streamline old-kernel fallback path of pipe2 to use syscalls directly

also, don't waste code/time on F_GETFL since pipes always have blank
flags initially (at least on old kernels, which are all this fallback
code matters for).
overhaul posix_spawn to use CLONE_VM instead of vfork

the proposed change was described in detail in detail previously on
the mailing list. in short, vfork is unsafe because:

1. the compiler could make optimizations that cause the child to
clobber the parent's local vars.

2. strace is buggy and allows the vforking parent to run before the
child execs when run under strace.

the new design uses a close-on-exec pipe instead of vfork semantics to
synchronize the parent and child so that the parent does not return
before the child has finished using its arguments (and now, also its
stack). this also allows reporting exec failures to the caller instead
of giving the caller a child that mysteriously exits with status 127
on exec error.

basic testing has been performed on both the success and failure code
paths. further testing should be done.
fix unsigned comparison bug in posix_spawn

read should never return anything but 0 or sizeof ec here, but if it
does, we want to treat any other return as "success". then the caller
will get back the pid and is responsible for waiting on it when it
immediately exits.
base system() on posix_spawn

this avoids duplicating the fragile logic for executing an external
program without fork.