Add regType verifier

[ruurdadema]

When registering a service, the regType must follow certain rules:

The service type followed by the protocol, separated by a dot
(e.g. "_ftp._tcp"). The service type must be an underscore, followed
by 1-15 characters, which may be letters, digits, or hyphens.
The transport protocol must be "_tcp" or "_udp".

It would be great to add a small verifier utility function to make sure users of the library (me) don't make mistakes using this.