PR/51805: Ngie Cooper: Fix file descriptor leaks

branches: 1.2.12;
fexecve is not implemented, so mark the test as an expected failure.

While doing this, clean this whole thing: do not define a useless cleanup
routine and wait for the subprocess to finish instead of using sleep.

branches: 1.1.2; 1.1.4;
Add most system calls for POSIX extended API set, part 2, with test cases:
faccessat(2), fchmodat(2), fchownat(2), fstatat(2), mkdirat(2), mkfifoat(2),
mknodat(2), linkat(2), readlinkat(2), symlinkat(2), renameat(2), unlinkat(2),
utimensat(2), openat(2).

Also implement O_SEARCH for openat(2)

Still missing:
- some flags for openat(2)
- fexecve(2) implementation