Add cname-domain override and lowercase normalization to dhcp-to-dns

[witul]

Two improvements to dhcp-to-dns:

1. Allow a separate domain for CNAME records (A records keep their
   MAC-based name in the technical domain, CNAMEs can resolve to a
   cleaner one). Configurable at three levels:

   • global: DhcpToDnsCnameDomain in global-config-overlay
   • per network: cname-domain= in DHCP network comment
   • per lease: cname-domain= in DHCP lease comment (highest priority)

2. Normalize generated DNS names to lowercase. DHCP clients often send
   mixed-case hostnames (e.g. DESKTOP-ABC123, iPhone-XYZ). A new
   ToLower helper in global-functions is used for both the MAC-based
   A record name and the hostname CNAME.

[eworm-de]

Hmm, why is this marked as draft? Anyway, found this interesting and reworked it a bit.

This is the current state, currently in staging branch: https://github.com/eworm-de/routeros-scripts/commits/1833e72a0060f02e2cfc8795b45f5a34237faca1

• split this a little more to separate the changes, for example having the conversion to lower case in its own commit
• reordered a bit, the conversion to lower case is not applied to full name
• dropped to global configuration ($DhcpToDnsCnameDomain) - I think that cases more confusion than it helps... Is that ok with you?

Feedback welcome... An thanks a lot for your contribution!

[eworm-de]

Oh, just remembered... We can use :convert with transform=lc. Will update later...