Request: add `exit` alias to `vvp`'s terminal interface

[parker-research]

When running vvp to execute a file, pressing Ctrl+C puts you into a terminal-like interface with the following commands:

> help
Commands can be from the following table of base commands,
or can be invocations of system tasks/functions.

cd       - Synonym for push.
cont     - Resume (continue) the simulation
finish   - Finish the simulation.
help     - Get help.
list     - List items in the current scope.
load     - Load a VPI module, a la vvp -m.
ls       - Shorthand for "list".
pop      - Pop one scope from the scope stack.
push     - Descend into the named scope.
step     - Single-step the scheduler for 1 event.
time     - Print the current simulation time.
trace    - Control statement tracing (on/off) when the code is instrumented.
where    - Show current scope, and scope hierarchy stack.

...

Adding an exit alias for finish would be very helpful so that it aligns with a standard *nix approach to "getting out of tools when at a prompt".

Additionally, Ctrl+C and Ctrl+D at this prompt should both probably do the finish action (that is, kill vvp).

[caryr]

Adding an alias should be relatively simple. Switching the signal handling at the prompt is possible, just needs to be handled correctly so they only have the finish functionality at the prompt.

[caryr]

I've added the alias for exit to development. Tweaking the signals needs more work.