fs_mgr: remount: add partition argument list

Can supply a specific partition to remount.  Partitions can be
specified by name or mount point.  Some extra work to differentiate
an unknown partition, invalid partition, or one that is covered by
overlayfs.

Test: adb-remount-test.sh
Bug: 122602260
Change-Id: Iab6f51c2b5ebe01f1cea3fb235445d5e2f495365