applet: create agent if --shell-debug is used (bgo #729334)

The applet agent wouldn't be created if --shell-debug was used, which
somewhat defeats the purpose of --shell-debug (which is to act like
nm-applet isn't running under gnome-shell for debugging purposes).

https://bugzilla.gnome.org/show_bug.cgi?id=729334
applet: don't block on ModemManager initialization (bgo #729334)

First, don't autostart ModemManager since the applet is only using
ModemManager for information and not for controlling modems.  Leave
that to system services (like systemd or upstart).  Requesting
autostart can block for long periods of time in some situations,
like if the process hangs during startup or isn't actually present
on the disk.

Second, make ModemManager initialization asynchronous to avoid any
other initialization blockage.  The applet only uses MM for info
about devices NM already knows about, so it's not critical to
applet startup.

https://bugzilla.gnome.org/show_bug.cgi?id=729334