Generic chainparam selection with -network=<strNetworkID>

Continues #4804.
Deprecate -testnet and -regtest in favor of -network=testnet and -network=regtest.
When the old args are used a warning will be logged.
The repetition of the invalid combination error string is also avoided.
Additionally, the chainparam networkID is removed since it's not used anymore (as it shouldn't).

I just copied the message that we had, but I actually prefer unknown or not supported.
I will change it to unknown.