path-util: rework find_binary(), fsck_exists() and mkfs_exists()

path-util: rework find_binary(), fsck_exists() and mkfs_exists()

Modernize the code a bit:

- Get rid of FOREACH_WORD_SEPARATOR() loop in favour of a
  extract_first_word() loop.

- Remove find_binary()'s "local" flag. It's not reasonably possible to
  look for binaries on remote systems, we hence should not pretend we
  could.

- When we cannot find a suitable binary, return the last error returned
  from access() rather than ENOENT unconditionally.

- Rework fsck_exists() and mkfs_exists() to return 1 on success, 0 if
  the implementation is missing and negative on real errors. This is
  more like we do it in other functions.

- Make sure we also detect direct fsck symlinks to "true", rather than
  just absolute ones to /bin/true.