patch by Pascal Cuoq (with minor tweaks to comments)

patch by Arvid Picciani (aep)

this is not required by the standard, but it's nicer than corrupting
the state and rather inexpensive.

note that none of these are implemented, and programs depending on
them may break... patch by sh4rm4

patches by sh4rm4, presumably needed to make gdb or some similar junk
happy...

it's a keyword in c++ (wtf). i'm not sure this is the cleanest
solution; it might be better to avoid ever defining __NEED_wchar_t on
c++. but in any case, this works for now.

this may be useful to posix_spawn..?

musl's dynamic linker does not support unloading dsos, so there's
nothing for this function to do. adding the symbol in case anything
depends on its presence..

mildly tested; may have bugs. the locking should be updated not to use
spinlocks but that's outside the scope of this one module.

the fcntl syscall can return a negative value when the command is
F_GETOWN, and this is not an error code but an actual value. thus we
must special-case it and avoid calling __syscall_ret to set errno.
this fix is better than the glibc fix (using F_GETOWN_EX) which only
works on newer kernels and is more complex.

no idea why these 4 are permuted and the rest are standard/generic

...and still be valid in #if directives.

right now it's questionable whether this change is an improvement or
not, but if we later want to support priority inheritance mutexes, it
will be important to have the code paths unified like this to avoid
major code duplication.

this is valid for error-checking mutexes; otherwise it invokes UB and
would be justified in crashing.

this simplifies the code paths slightly, but perhaps what's nicer is
that it makes recursive mutexes fully reentrant, i.e. locking and
unlocking from a signal handler works even if the interrupted code was
in the middle of locking or unlocking.

a reader unlocking the lock need only wake one waiter (necessarily a
writer, but a writer unlocking the lock must wake all waiters
(necessarily readers). if it only wakes one, the remainder can remain
blocked indefinitely, or at least until the first reader unlocks (in
which case the whole lock becomes serialized and behaves as a mutex
rather than a read lock).

mildly tested, seems to work

passing null pointer for %s is UB but lots of broken programs do it anyway

there is no need to send a wake when the lock count does not hit zero,
but when it does, all waiters must be woken (since all with the same
sign are eligible to obtain the lock).

seeking back can be performed by the caller, but if the caller doesn't
expect it, it will result in an infinite loop of failures.

eliminate the sequence number field and instead use the counter as the
futex because of the way the lock is held, sequence numbers are
completely useless, and this frees up a field in the barrier structure
to be used as a waiter count for the count futex, which lets us avoid
some syscalls in the best case.

as of now, self-synchronized destruction and unmapping should be fully
safe. before any thread can return from the barrier, all threads in
the barrier have obtained the vm lock, and each holds a shared lock on
the barrier. the barrier memory is not inspected after the shared lock
count reaches 0, nor after the vm lock is released.

i think this works, but it can be simplified. (next step)

the vm lock only waits for threads in the same process exiting.
actually this fix is not enough, but it's a start...

it was assuming the result of the condition it was supposed to be
checking for, i.e. that the thread ptr had already been initialized by
pthread_mutex_lock. use the slower call to be safe.

we're not required to check this except for error-checking mutexes,
but it doesn't hurt. the new test is actually simpler/lighter, and it
also eliminates the need to later check that pthread_mutex_unlock
succeeds.