configure.ac

A slightly different approach. In terms of syscalls it probably doesn't
matter at all, but it's a simpler, less hacky and maybe more efficient
way to work around PATH_MAX limitations than constantly chdir()'ing for
each directory.  Should also fix the chdir("..") fail case in MacOS
firmlink scenarios, though I haven't a clue if MacOS implements all
these openat()-related calls in the first place.

Downsides:
- Less portable. Everything is in POSIX, but it's more recent than what
  we used to use.
- Requires more open file descriptions, might hit the limit for deeply
  nested directories.

Haven't done much testing yet, but there's a bunch of TODO's:
- Make refreshing work again (not sure what's going wrong there)
- See if we can report readdir() etc errors again?
- Make firmlink detection work again?
- See how portable this really is.
- Also update file deletion code