Previously, the default for acceptnonstdtxn defaulted to 0 on all
chains except testnet. Change this to be consistent across all
chains, and remove the parameter from chainparams entirely.
This commit effectively moves the definition of these constants
out of the chainparamsbase to their own file.
Using the ChainType enums provides better type safety compared to
passing around strings.
The commit is part of an ongoing effort to decouple the libbitcoinkernel
library from the ArgsManager and other functionality that should not be
part of the kernel library.
Moves chainparams code not using the ArgsManager to the kernel.
Subsequently use the kernel chainparams header now where possible in
order to further decouple chainparams call sites from gArgs.
