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).

That's what it does, look at the last return.
I use "" as the default for the later (network != "") check to return an error.