nixos: Require networking.hostName to be a valid DNS label

This also means that the hostname must not contain the domain name part
anymore (i.e. must not be a FQDN).
See RFC 1035 [0], "man 5 hostname", or the kernel documentation [1].
Note: For legacy reasons we also allow underscores inside of the label
but this is not recommended and intentionally left undocumented.

[0]: https://tools.ietf.org/html/rfc1035
[1]: https://www.kernel.org/doc/html/latest/admin-guide/sysctl/kernel.html#domainname-hostname



Co-authored-by: zimbatm <zimbatm@zimbatm.com>