Add an IO function to call subprocesses

[developedby]

Is your feature request related to a problem? Please describe.
A lot of IO things we want to do are already available as commands in the systems where hvm is running.
Sure, we'd like to be able to do all that inside of HVM, but implementing all the wrappers takes a long time.
Also, some things (proprietary, non-source available software) can only be run through their binaries.
So, we'd like a way to call binaries in the system.

Describe the solution you'd like
An IO function that runs a command in the system it's running. Similar to C's system.
preferably with a way to capture stdin, stdout and stderr.
I think it should be blocking.

Describe alternatives you've considered

• Something like execl where we run binaries directly instead of calling shell commands.
• Not having this at all. I don't think this is really a viable option long term.