Chain-selection from UI (#4859)

* First little bits for chain-selection.

* Provide RPCs and get settings through to user defaults.

* Hasty stash.

* Fix updater accidentally redownloading.

* Finish up.

* Add JS tests.

* Hypervisor should never run a binary modified before itself.

* Style.

* Help tweak.

* Fix test compile.

* Fix JS test

* Build fix for tests.

* Revert default chain name

* Another test

* Use spec name via client.

* Fix mock up.

* whitespace

[ci:skip]

* whitespace

[ci:skip]

* remove exit/restart endpoints.