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'd use ``if (network.empty() || network == "main")`` that way the last case can be removed.