Include PoS NetUpgrades in RegTest when specified from the command line

[alfiedotwtf]

This will be needed for #1004

[iljakuklic]

I am fine with this in principle. But agree with @azarovh that some of the code duplication should be addressed and it should also be relatively easy to replicate the behaviour of the command line switch using a simple method call (on a builder or something like that) so we can get the same setting in unit tests too.

[alfiedotwtf]

But agree with @azarovh that some of the code duplication should be addressed

Yep, agreed. Fixed.

it should also be relatively easy to replicate the behaviour of the command line switch using a simple method call (on a builder or something like that) so we can get the same setting in unit tests too.

To make pos-netupgrades an accessor to Builder I would need to add it to ChainConfigBuilder, which would then be seen to all chain types - something that should probably be avoided given we only want this flag in RegTest. As for getting the same setting under unit tests, ChainConfigOptions has all fields as pub so if need be it can be accessed there.

[TheQuantumPhysicist]

Unify the impl blocks and rename to regtest. Otherwise this looks good.