unlocking an unlocked mutex is not UB for robust or error-checking
mutexes, so we must avoid calling __pthread_self (which might crash
due to lack of thread-register initialization) until after checking
that the mutex is locked.

this roughly halves the cost of pthread_mutex_unlock, at least for
non-robust, normal-type mutexes.

the a_store change is in preparation for future support of archs which
require a memory barrier or special atomic store operation, and also
should prevent the possibility of the compiler misordering writes.

cycle-level benchmark on atom cpu showed typical pthread_mutex_lock
call dropping from ~120 cycles to ~90 cycles with this change. benefit
may vary with compiler options and version, but this optimization is
very cheap to make and should always help some.

- there is no longer any risk of spoofing cancellation requests, since
  the cancel flag is set in pthread_cancel rather than in the signal
  handler.

- cancellation signal is no longer unblocked when running the
  cancellation handlers. instead, pthread_create will cause any new
  threads created from a cancellation handler to unblock their own
  cancellation signal.

- various tweaks in preparation for POSIX timer support.

actually this trick also seems to have made the uncontended case slower.

glibc made the ridiculous choice to use pass-by-register calling
convention for these functions, which is impossible to duplicate
directly on non-gcc compilers. instead, we use ugly asm to wrap and
convert the calling convention. presumably this works with every
compiler anyone could potentially want to use.

this commit addresses two issues:

1. a race condition, whereby a cancellation request occurring after a
syscall returned from kernelspace but before the subsequent
CANCELPT_END would cause cancellable resource-allocating syscalls
(like open) to leak resources.

2. signal handlers invoked while the thread was blocked at a
cancellation point behaved as if asynchronous cancellation mode wer in
effect, resulting in potentially dangerous state corruption if a
cancellation request occurs.

the glibc/nptl implementation of threads shares both of these issues.

with this commit, both are fixed. however, cancellation points
encountered in a signal handler will not be acted upon if the signal
was received while the thread was already at a cancellation point.
they will of course be acted upon after the signal handler returns, so
in real-world usage where signal handlers quickly return, it should
not be a problem. it's possible to solve this problem too by having
sigaction() wrap all signal handlers with a function that uses a
pthread_cleanup handler to catch cancellation, patch up the saved
context, and return into the cancellable function that will catch and
act upon the cancellation. however that would be a lot of complexity
for minimal if any benefit...

with this patch, the syscallN() functions are no longer needed; a
variadic syscall() macro allows syscalls with anywhere from 0 to 6
arguments to be made with a single macro name. also, manually casting
each non-integer argument with (long) is no longer necessary; the
casts are hidden in the macros.

some source files which depended on being able to define the old macro
SYSCALL_RETURNS_ERRNO have been modified to directly use __syscall()
instead of syscall(). references to SYSCALL_SIGSET_SIZE and SYSCALL_LL
have also been changed.

x86_64 has not been tested, and may need a follow-up commit to fix any
minor bugs/oversights.

this commit shuffles around the location of syscall definitions so
that we can make a syscall() library function with both SYS_* and
__NR_* style syscall names available to user applications, provides
the syscall() library function, and optimizes the code that performs
the actual inline syscalls in the library itself.

previously on i386 when built as PIC (shared library), syscalls were
incurring bus lock (lock prefix) overhead at entry and exit, due to
the way the ebx register was being loaded (xchg instruction with a
memory operand). now the xchg takes place between two registers.

further cleanup to arch/$(ARCH)/syscall.h is planned.

some of this code should be cleaned up, e.g. using macros for some of
the bit flags, masks, etc. nonetheless, the code is believed to be
working and correct at this point.

if the mutex was previously locked, we can assume pthread_self was
already called at the time of locking, and thus that the thread
pointer is initialized.

this change is necessary to free up one slot in the mutex structure so
that we can use doubly-linked lists in the implementation of robust
mutexes.

for some reason these functions are not shaded by the PS/TPS option in
POSIX, so presumably they are mandatory, even though the functionality
they offer is optional. for now, provide them in case any programs
depend on their existence, but disallow any priority except the
default.

multiple opens of the same named semaphore must return the same
pointer, and only the last close can unmap it. thus the ugly global
state keeping track of mappings. the maximum number of distinct named
semaphores that can be opened is limited sufficiently small that the
linear searches take trivial time, especially compared to the syscall
overhead of these functions.

we can avoid blocking signals by simply using a flag to mark that the
thread has exited and prevent it from getting counted in the rsyscall
signal-pingpong. this restores the original pthread create/join
throughput from before the sigprocmask call was added.

it must return errno, not -1, and should reject invalud values for how.

the set_tid_address returns the tid (which is also the pid when called
from the initial thread) so there is no need to make a separate
syscall to get pid/tid.

the issue was a break statement that was breaking only from the
switch, not the enclosing for loop, and a failure to set the final
success state.

problem 1: mutex type from the attribute was being ignored by
pthread_mutex_init, so recursive/errorchecking mutexes were never
being used at all.

problem 2: ownership of recursive mutexes was not being enforced at
unlock time.