Add indicate-null-switch config option.

Allows disabling/enabling switch-indicator for switch operations
that don't actually switch the active node (or only doing so for
hotkey-triggered switches).
Clean up master's handle_fds().

Get rid of extraneous calls to perform scheduled events and refactor
next-event-time into a separate function.
Introduce & use new for_each_remote() control macro.
Add hotkey-bindable 'quit' action.

Mostly just to get a clean exit path for valgrind leak checks.
Clean up some minor memory leaks.

Nothing major, basically just freeing stuff allocated during
parse_cfg() and a small number of other O(1) things.  Master appears
clean (leak-free) through valgrind, with the exception some space
allocated by XCreatePixmapFromBitmapData() that for some reason
doesn't seem to get freed by XFreePixmap().