improve signal handling

[karmacoma-eth]

when halmos is waiting on a solver and we type ctrl+c, we don't want the current solver to get the signal, we want halmos to get the signal and interrupt the run cleanly (killing already spawned solvers).

Something like this:

import os
import subprocess
import sys
import atexit

# Create process with different flags based on platform
if os.name == 'nt':  # Windows
    proc = subprocess.Popen(
        ["jsi", "--full-run", "examples/unsat_div.smt2"],
        creationflags=subprocess.CREATE_NEW_PROCESS_GROUP
    )
else:  # Unix-like
    os.setpgrp()
    proc = subprocess.Popen(
        ["jsi", "--full-run", "examples/unsat_div.smt2"],
        preexec_fn=os.setpgrp,
        start_new_session=True
    )

print(f"started {proc.pid=}")
atexit.register(lambda: proc.kill())
sys.exit(42)

Explanation: all processes in the foreground process group of the current session receive the signal, so we want the external solvers to run in a different session and process group, in order to make sure that halmos itself orchestrates external processes correctly.