libmount: allow unspecified source on remount

kernel does not require mount source (e.g. device name) on remount, it
means that fstab/mtab/mountinfo should be optional in this case.
For example:

 mount -o rw,remount /

has to work on system without mounted /proc.

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