Code needed in a modern patch:

1) Use the system call if compile time configure detected it may be available.  (if we just use syscall() rather than a libc wrapper, a configure check may not be necessary, but various #ifdefs likely are)
2) If (1) produces an error saying it isn't available at runtime (ie: running on a kernel that doesn't support it, regardless of where we were built), fall back to another approach (3)
3) if iterating over /proc/$pid/fd works at runtime, use that; else (4)
4) the existing brute force code.