Bug 670928 - cmdline ignored if Tomboy is running

Command line arguments get ignored if Tomboy is already running.
This is caused by named mutexes which, since Mono 2.8 [1], work only if
the MONO_ENABLE_SHM environment variable is set.

I assume this setting was changed for a good reason and we now use DBus
for single-instance detection.

[1] http://www.mono-project.com/Release_Notes_Mono_2.8#Shared_handles_are_disabled_by_default



Signed-off-by: Jared Jennings <jjennings@src.gnome.org>