libmount: merge utab into listmount-based tables

libmount: merge utab into listmount-based tables

Extract utab merge logic from __mnt_table_parse_mountinfo() into a
shared mnt_table_merge_utab() function and call it at the end of
mnt_table_fetch_listmount(). This makes userspace mount options
(x-systemd.*, user=, etc.) available when the mount table is obtained
via listmount/statmount syscalls.

The merge sets user_optstr directly (instead of the combined optstr)
to avoid conflicts with lazy statmount() which populates kernel
options on demand.

Add mnt_table_disable_useropts() public API to allow callers to skip
utab merging when only kernel-side data is needed.

Use the new API in findmnt --kernel=listmount to keep backward
compatible behavior (kernel-only data); users who want userspace
options can use --mtab.

Signed-off-by: Karel Zak <kzak@redhat.com>
(cherry picked from commit c839a9ac8fed409c332c94ec69ddc11aea97ae80)