libmount: fix and improve utab update on MS_MOVE

libmount: fix and improve utab update on MS_MOVE

* avoid double '//'

* don't update /fooxxx when /foo update requested (make sure that
  startswith() returns path terminated by '/')

* canonicalize only once the new path -- all in utab/mtab is already
  canonicalized, so after MS_MOVE we need to care about the new path
  only

* use asprintf() rather than strcpy() and strcat(), don't compose a
  new path from prefix and subdir when replace entire path

Addresses: https://github.com/util-linux/util-linux/pull/1660
Signed-off-by: Karel Zak <kzak@redhat.com>