fix special-case breakage in popen due to reversed argument order

rewrite popen to use posix_spawn instead of fragile vfork hacks

clean up sloppy nested inclusion from pthread_impl.h

this mirrors the stdio_impl.h cleanup. one header which is not
strictly needed, errno.h, is left in pthread_impl.h, because since
pthread functions return their error codes rather than using errno,
nearly every single pthread function needs the errno constants.

in a few places, rather than bringing in string.h to use memset, the
memset was replaced by direct assignment. this seems to generate much
better code anyway, and makes many functions which were previously
non-leaf functions into leaf functions (possibly eliminating a great
deal of bloat on some platforms where non-leaf functions require ugly
prologue and/or epilogue).

fix copy/paste error in popen changes that broke signals

signal mask was not being restored after fork, but instead blocked again.

fix usage of locks with vfork

__release_ptc() is only valid in the parent; if it's performed in the
child, the lock will be unlocked early then double-unlocked later,
corrupting the lock state.

overhaul system() and popen() to use vfork; fix various related bugs

since we target systems without overcommit, special care should be
taken that system() and popen(), like posix_spawn(), do not fail in
processes whose commit charges are too high to allow ordinary forking.

this in turn requires special precautions to ensure that the parent
process's signal handlers do not end up running in the shared-memory
child, where they could corrupt the state of the parent process.

popen has also been updated to use pipe2, so it does not have a
fd-leak race in multi-threaded programs. since pipe2 is missing on
older kernels, (non-atomic) emulation has been added.

some silly bugs in the old code should be gone too.

make popen cancellation-safe

close was the only cancellation point called from popen, but it left
popen with major resource leaks if any call to close got cancelled.
the easiest, cheapest fix is just to use a non-cancellable close
function.

popen: handle issues with fd0/1 being closed

also check for failure of dup2 and abort the child rather than
reading/writing the wrong file.

initial check-in, version 0.5.0