'onenv' program to inject tasks into specific environments

[fachat]

Similarly to the "ontty" program there should be an "onenv" program to run a task in a specific (pre-existing) environment.

This would allow injecting a monitor program into the runtime/environment of an existing program for example.
Other uses would be to put programs into the background inside the system bank of the c128 (where there is quite some space currently unused)