main: add --debug, fix logging under systemd

main: add --debug, fix logging under systemd

When run with --no-daemon, NM used to duplicate all syslog output to
stderr, for ease of debugging. But this meant it had to tell systemd
to ignore stderr, so you wouldn't get duplicated log entries. But that
meant we lost error messages that didn't go through nm_log. (eg,
g_warning()s and g_return_if_fail()s).

Fix this by making --no-daemon no longer duplicate syslog output to
stderr, and removing the "StandardError=null" from the systemd service
file. To get the old behavior, you can use --debug instead of
--no-daemon.

https://bugzilla.gnome.org/show_bug.cgi?id=700550