fshelp: Add handling of "." and ".." and grub_fshelp_find_file_lookup.

fshelp: Add handling of "." and ".." and grub_fshelp_find_file_lookup.

Recent tests have discovered that many of our filesystems have flawed
handling of "." and "..". Rather than attempting to fix it in filesystems
themselves, make the common code fshelp aware of "." and ".." and handle
them in this layer. Add grub_fshelp_find_file_lookup for easy conversion
of BFS, HFS and exFAT which have the same problem and don't use fshelp.