media-keys: Merge power_action_* functions

They only differed by a single line.
media-keys: Make the "shutdown" action not be interactive

Shutdown is supposed to shutdown without asking, as "interactive"
already should do what we need for interactive shutdowns.

https://bugzilla.gnome.org/show_bug.cgi?id=698733
media-keys: Allow the power key in more places

We allow using the power key pretty much everywhere now, but
we also ensure that we never show interactive dialogues on the
lock screen (something that was possible with other suspend/power
buttons already if more than more than one user was logged in,
or an interactive shutdown was configured).

See https://bugzilla.redhat.com/show_bug.cgi?id=980692

https://bugzilla.gnome.org/show_bug.cgi?id=707095