libmount: (--all) continue although /proc is not mounted

libmount: (--all) continue although /proc is not mounted

Now 'mount --all' ends with error if /proc is not mounted and there is
some other entry before /proc in fstab. This commit improves this
situation and ignores all mount table related errors if the table is
empty.

This is important for situation when there is for example "/" as the
first line in fstab.

Addresses: https://github.com/util-linux/util-linux/issues/1492
Signed-off-by: Karel Zak <kzak@redhat.com>