ctlmgr/gui/master: start subprocesses in new pgroup

This only makes a difference on POSIX. It prevents subprocesses
from receiving the signals that the parent receives. For ctlmgr
and master is cuts down on spam on the console (KeyboardInterrupt
tracebacks from all controllers) and enforces that proper
termination is followed.

This does not help if the parent gets SIGKILL (subprocesses
may linger).

artiq/devices/ctlmgr.py

@@ -86,7 +86,7 @@ class Controller:
                     self.process = await asyncio.create_subprocess_exec(
                         *shlex.split(self.command),
                         stdout=subprocess.PIPE, stderr=subprocess.PIPE,
-                        env=env)
+                        env=env, start_new_session=True)
                     asyncio.ensure_future(
                         LogParser(self._get_log_source).stream_task(
                             self.process.stdout))

artiq/gui/applets.py

@@ -116,7 +116,8 @@ class AppletDock(QDockWidgetCloseDetect):
         )
         logger.debug("starting command %s for %s", command, self.applet_name)
         try:
-            await self.ipc.create_subprocess(*shlex.split(command))
+            await self.ipc.create_subprocess(*shlex.split(command),
+                                             start_new_session=True)
         except:
             logger.warning("Applet %s failed to start", self.applet_name,
                            exc_info=True)

artiq/master/worker.py

@@ -87,7 +87,7 @@ class Worker:
                 sys.executable, "-m", "artiq.master.worker_impl",
                 self.ipc.get_address(), str(log_level),
                 stdout=subprocess.PIPE, stderr=subprocess.PIPE,
-                env=env)
+                env=env, start_new_session=True)
             asyncio.ensure_future(
                 LogParser(self._get_log_source).stream_task(
                     self.ipc.process.stdout))