fstyp(8): Show exFAT volume labels with -l flag

exfat is fundamentally the same design as fat32. The superblock differs
marginally, and there are some additional optional features irrelevant to
fstype(8); the structure of dirents has changed slightly to enable, among
other things, larger files; the directory entries are no longer DOS 8.3
ASCII or local 8-bit encoding, but instead explicitly UCS-2-LE.

(As a result, this change uses iconv to convert a found exfat volume label
to the user's locale.)

Locating the volume label is identical to FAT32: locate the root directory
and walk through dirents until you find a volume label. Like FAT32, follow
the FAT chain between root directory clusters as necessary.

PR: 242225
Reported by: Victor Sudakov <vas AT sibptus.ru>