Fix clamdscan configuration handling

 - Honor configured 'false' values
 - Treat 'scan_all' consistently with other options--permitting 0, 1, 'yes', or 'no' as values
Working toward functional virus/clamdscan tests.
More complete clamdcscan tests
 * construct our own Qpsmtpd::Transaction object for testing, so we're sure
   to have a pristine state
 * Move some logic into a should_scan() sub, to help separate scan_all from
   is_not_multipart (since the multipart state of a message has nothing to
   do with any configuration parameters)
Style cleanup

 * no more 'return ()'
 * Explicit 'return 0'
Reverse sense of is_not_multipart to is_multipart.
New tests to cover new handling of register-time configuration.
Merge pull request #104 from flimzy/clam-opts

Fix clamdscan configuration handling