Allow appending to the service environment.

The new registry value AppEnvironmentExtra allows defining a list of
environment variables which will be appended to the environment, as
opposed to those defined by the srvany-compatible AppEnvironment, which
replaces the environment.

Though it would serve no useful purpose beyond verifying whether or not
I went to the effort of writing a code path for it, it is possible to
define bother lists and have them behave intuitively.