Fix compile error w/ DISABLE_GUI
Add FIFO to send commands to kalu; Closes #9

On start a FIFO named kalu_fifo_XXXX (where XXXX is kalu's process id) will be
created under $XDG_RUNTIME_DIR. One can write to this FIFO to send commands to
kalu, to e.g. run checks, start a sysupgrade, etc

This allows to use kalu without the need to click its systray icon/menu, even
from keyboard shortcuts.