1. failed match of literal chars from the format string would always
return matching failure rather than input failure at eof, leading to
infinite loops in some programs.

2. unread of eof would wrongly adjust the character counts reported by
%n, yielding an off-by-one error.

this is explicitly allowed by POSIX

some functions that should have been testing whether pthread_self()
had been called and initialized the thread pointer were instead
testing whether pthread_create() had been called and actually made the
program "threaded". while it's unlikely any mismatch would occur in
real-world problems, this could have introduced subtle bugs. now, we
store the address of the main thread's thread descriptor in the libc
structure and use its presence as a flag that the thread register is
initialized. note that after fork, the calling thread (not necessarily
the original main thread) is the new main thread.

the linux documentation for dup2 says it can fail with EBUSY due to a
race condition with open and dup in the kernel. shield applications
(and the rest of libc) from this nonsense by looping until it succeeds

the check against MADV_DONTNEED to because linux MADV_DONTNEED
semantics conflict dangerously with the POSIX semantics

this also de-uglifies the dummy function aliasing a bit.

these functions are allowed to be cancellation points, but then we
would have to install cleanup handlers to avoid termination with locks
held.

we already checked before making the syscall, but it's possible that a
signal handler interrupted the blocking syscall and disabled
cancellation, and that this is the cause of EINTR. in this case, the
old behavior was testably wrong.

like all other syscalls, close should return to the caller if and only
if it successfully performed its action. it is necessary that the
application be able to determine whether the close succeeded.

clean and simple, but fails when the caller does not have permissions
to open the file for reading or when /proc is not available. i may
replace this with a full implementation later, possibly leaving this
version as an optimization to use when it works.

if the exit was caused by cancellation, __cancel has already set these
flags anyway.

cancellation frames were not correctly popped, so this usage would not
only loop, but also reuse discarded and invalid parts of the stack.

don't waste time (and significant code size due to function call
overhead!) setting errno when the result of a syscall does not matter
or when it can't fail.