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).

Actually I wanted to check network before checking testnet/regtest so that if you provide both network="regtest" and -testnet, the network arg has preference and the -testnet arg is ignored. But doing it in the other order allows this simplification now and produces less changes when those flags are disabled so I'll change it.