orc shell: exec rather than invoking subprocess

[mrjackv]

This commit does 2 things:

• Introduces an additional parameter to _run_script called exec_, if set to true it will make the script be exec'ed instead of running it in a subprocess
• Makes use of the change above in the subcommand orc shell that from now on will exec the target executable rather than invoke a subshell, this fixes a few problems with terminal and signal handling.

I've also tweaked the typing annotations in the affected functions.

[aleclearmind]

Given our recent experience with temporary files and exec, I wonder if this is a good idea. Here they suggest to fork, let the forked process shutdown and execve in the old process. What do you think?

[mrjackv]

The basic problem is that, without adding a lot of extra logic, the best way to do orc shell is to exec, double-forking would only complicate things and not solve our problem. I checked the codebase and there are only two places where a temporary file is used, and in both cases they're used with a context manager (with), so the only way for a file leak would be to exec within the with (which does not happen since in both occasions we are inside the context for very little time) Also, even if there was an instance of file leakage I'd prefer to fix it (because "morally" the tempfile is extended beyond its actual lifetime) rather handle the alternative fix for orc shell

[mrjackv]

@aleclearmind made exec fork to free resources