Close children processes gracefully upon SIGINT (and other termination signals)

[rbonichon]

          I don't know if it is expected, but we get:

t=2023-12-07T19:34:31+0100 lvl=eror msg="failed to close pre-image server" err="signal: interrupt"

when stopping the zkVM with CTRL-C.

Not urgent IMHO, and definitely not in the scope of this PR, but would be nice to open a follow-up.

Originally posted by @dannywillems in #1469 (comment)