libmount: compose mount.<type> helpers paths more carefully

The [u]mount helper path is "/sbin/mount.<type>", the <type> should not
be possible to interpret like a path, for example:

  # mkdir /sbin/mount.fake
  # mount -t fake/../../../home/user/foo.sh

Anyway, this is *not mount(8) security issue*, because:

  - non-root users don't have permissions to specify -t on mount(8)
    command line
  - /sbin is not writable for regular users

This patch makes libmount more robust for situations when the library
is used by incompetent developers who don't sanitize user's input.

Reported-by: "Horsfall, Matthew" <mhorsfal@akamai.com>
Signed-off-by: Karel Zak <kzak@redhat.com>