Do not create new Unix session for subprocesses

This fixes a bug where subprocesses would not be terminated upon
termination of `litani run-build`.

Prior to this commit, every Litani job would run its command in a fresh
Unix session. This disconnected the command from the controlling
terminal, such that a signal delivered from the terminal would not reach
the command. Thus, users who sent INT to `litani run-build` by pressing
Ctrl-C would observe that `litani run-build`, `ninja`, and all of the
`litani exec` invocations would terminate, but the job commands would be
reparented to init(1) and continue to run with no controlling terminal.
This commit ensures that job commands once again receive the INT, TERM,
and KILL signals.

This reverts commit 241c5e8.

Author: karkhaz

lib/process.py

@@ -20,7 +20,6 @@
 import logging
 import os
 import platform
-import signal
 import subprocess
 import sys
 
@@ -219,19 +218,18 @@ async def __call__(self):
 
         proc = await asyncio.create_subprocess_shell(
             self.command, stdout=asyncio.subprocess.PIPE, stderr=pipe,
-            cwd=self.cwd, env=env, start_new_session=True)
+            cwd=self.cwd, env=env)
         self.proc = proc
 
         timeout_reached = False
         try:
             out, err = await asyncio.wait_for(
                 proc.communicate(), timeout=self.timeout)
         except asyncio.TimeoutError:
-            pgid = os.getpgid(proc.pid)
-            os.killpg(pgid, signal.SIGTERM)
+            proc.terminate()
             await asyncio.sleep(1)
             try:
-                os.killpg(pgid, signal.SIGKILL)
+                proc.kill()
             except ProcessLookupError:
                 pass
             out, err = await proc.communicate()