autostart: validate autostart condition gsettings key

gnome-session will crash if fed a desktop file with an
autostart condition that references an invalid key for
a given schema.

This commit validates that the key associated with the
autostart condition is defined in the schema and defined
to have a boolean type.

https://bugzilla.gnome.org/show_bug.cgi?id=740119
Add new commandline option - disable-acceleration-check

Some other lines were moved around so that debug logging will work
at the time of acceleration check.