Fix hostname validation for nameservers

Fix hostname validation for nameservers

Fixes the hostname validation to align with the RFC's demands[1].
This was done by replacing the full regex with a function that
broke the FQDN into individual components that were easier to
reason about with regular expressions.

Also added several test cases for domains so if someone wants
to convert it back to pure regex there will be better test vectors.

1. RFC 1123 says an all-digit hostname is allowed in section 2.1. It
   says that this more liberal syntax MUST be supported.

Closes-Bug: #1396932
Change-Id: I003cf14d95070707e43e40d55da62e11a28dfa4e