Wire up most of the certificate validation code for OS X. Still need to

Wire up most of the certificate validation code for OS X.  Still need to
finish the keychain and subject alternate name changes, but we now have the
core validation framework in place.

Also need to decide whether to allow requests to untrusted connections by
default or require all applications to copy and validate manually.

- Updated httpCredentialsAreTrusted to require the common name.
- Implementation of httpCredentialsAreTrusted, httpCredentialsGetExpiration,
  httpCredentialsIsValidName, and httpCredentialsString.
- Added display of credential info to testhttp program.

git-svn-id: svn+ssh://src.apple.com/svn/cups/cups.org/trunk@11829 a1ca3aef-8c08-0410-bb20-df032aa958be