md5_verify() can now verify several files at once

This also makes the output just a wee bit nicer :)

e.g.:

  comparing MD5 checksums:
    ./packages/seamonkey-solibs-2.17.1-i486-1_slack14.0.txz: OK
    ./packages/seamonkey-solibs-2.17.1-i486-1_slack14.0.txz.asc: OK

vs.

  comparing MD5 checksums for file `xyz1':
    ./xyz1: OK
  comparing MD5 checksums for file `xyz2':
    ./xyz2: OK