fix uninitialized mode variable in openat function

this was introduced in commit 2da3ab1382ca8e39eb1e4428103764a81fba73d3
as an oversight while making the variadic argument access conditional.

fix invalid access by openat to possibly-missing variadic mode argument

the mode argument is only required to be present when the O_CREAT or
O_TMPFILE flag is used.

include cleanups: remove unused headers and add feature test macros

overhaul pthread cancellation

this patch improves the correctness, simplicity, and size of
cancellation-related code. modulo any small errors, it should now be
completely conformant, safe, and resource-leak free.

the notion of entering and exiting cancellation-point context has been
completely eliminated and replaced with alternative syscall assembly
code for cancellable syscalls. the assembly is responsible for setting
up execution context information (stack pointer and address of the
syscall instruction) which the cancellation signal handler can use to
determine whether the interrupted code was in a cancellable state.

these changes eliminate race conditions in the previous generation of
cancellation handling code (whereby a cancellation request received
just prior to the syscall would not be processed, leaving the syscall
to block, potentially indefinitely), and remedy an issue where
non-cancellable syscalls made from signal handlers became cancellable
if the signal handler interrupted a cancellation point.

x86_64 asm is untested and may need a second try to get it right.

global cleanup to use the new syscall interface

initial check-in, version 0.5.0