stat: support more device number representations

stat: support more device number representations

In preparation for changing the default device number
representation (to decomposed decimal), provide more
formatting options for device numbers.

These new (FreeBSD compat) formatting options are added:

   %Hd  major device number in decimal (st_dev)
   %Ld  minor device number in decimal (st_dev)
   %Hr  major device type in decimal (st_rdev)
   %Lr  minor device type in decimal (st_rdev)
   %r   (composed) device type in decimal (st_rdev)
   %R   (composed) device type in hex (st_rdev)

* doc/coreutils.texi (stat invocation): Document new formats.
* src/stat.c (print_it): Handle the new %H and %L modifiers.
(print_statfs): Adjust to passing the format as two chars
rather than an int.  Using an int was introduced in commit db42ae78,
but using separate chars is cleaner and more extensible.
(print_stat): Likewise.  Handle any modifiers and the new 'r' format.
(usage): Document the new formats.
* tests/misc/stat-fmt.sh: Add a test case for new modifiers.
Addresses https://bugs.gnu.org/48960