libmount: add reference counting to libmount_fs

libmount: add reference counting to libmount_fs

 * mnt_new_fs() returns object with refcount=1
 * mnt_free_fs() does not care about reference counter

 * new functions mnt_ref_fs() and mnt_unref_fs()

 * mnt_table_add_fs() and mnt_table_rem_fs() uses reference counter

 * libmmnt_context uses reference counter for internal FS (as it could be
   shared outside the context)

 * backwardly incompatible change:

	- FS could be deallocated after mnt_table_remove_fs()

 * it's recommended to use mnt_unref_fs() after mnt_table_add_fs()

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