Add common DynDNS constants and code

to system.inc to remove code duplication
Use common code and constants from services.inc

Reduce code duplication
Use common code and constants from services.inc

to reduce code duplication
Use common code and constants from services.inc

to reduce code duplication
Remove leading spaces from DYNDNS_PROVIDER_DESCRIPTIONS

Remove leading spaces to make the string consistent.
Merge pull request #435 from phil-davis/master

Put common DynDNS code and constants into services.inc