scheduler: avoid waiting for stdout eof of /etc/network/ scripts

Scripts in /etc/network/ are executed using `exec_command` which
captures stdout by default, and thus waits for stdout end-of-file via
`Popen.communicate()`. However, this can cause hangs if the network
script executes a long-running command in the background. Can be
reproduced by putting the following (executable) script in
/etc/network/if-up.d/:

	#!/bin/sh
	sleep 5&

This script will cause `ifreload -a` to wait for 5 seconds per network
interface.

To avoid waiting, do not capture stdout when executing /etc/network/
scripts. This also improves compatibility with ifupdown, which runs
the above script in the background.

Signed-off-by: Friedrich Weber <f.weber@proxmox.com>

ifupdown2/ifupdown/scheduler.py

@@ -142,7 +142,7 @@ class ifaceScheduler():
             for mname in ifupdownobj.script_ops.get(op, []):
                 ifupdownobj.logger.debug("%s: %s : running script %s" % (ifacename, op, mname))
                 try:
-                    utils.exec_command(mname, env=command_env)
+                    utils.exec_command(mname, env=command_env, stdout=False)
                 except Exception as e:
                     if "permission denied" in str(e).lower():
                         ifupdownobj.logger.warning('%s: %s %s' % (ifacename, op, str(e)))