libmount: add mnt_table_parse_utab()

libmount: add mnt_table_parse_utab()

Add new public API to parse the utab file into a table. The utab path
and format are private library details; this function hides them from
callers. NULL as filename is recommended to let the library determine
the correct path.

This is needed by systemd for incremental fanotify-based mount
monitoring, where utab needs to be parsed independently of the full
mountinfo/listmount rescan path.

Signed-off-by: Karel Zak <kzak@redhat.com>