this bug seems to have caused any failure by pipe2 on such systems to
set errno to 1, rather than the proper error code.

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.

these interfaces have been adopted by the Austin Group for inclusion
in the next version of POSIX.

based on patch by orc and Isaac Dunham, with some details fixed.