Stop FQDN in hostname field

In various places the whole FQDN can be put in a field that requires just a hostname (the domain part has been asked for as a separate field).
Adds a new function is_unqualified_hostname($hostname)
Validate these host names to check they just contain a name without any "." bits:
- System hostname itself in the setup wizard and system, general setup
- host names in DNS host overrides
- host names in DHCP static map

This will prevent people doing things like this accidentally:
https://forum.pfsense.org/index.php?topic=78356.msg460062#msg460062