new module unix.signals: app-level signal handlers

new module unix.signals: app-level signal handlers
example:

IN: scratchpad USING: unix.ffi unix.signals ;
Loading resource:basis/unix/signals/signals.factor
IN: scratchpad [ V{ "Me not that kind of orc!" "Me busy, leave me alone!" "Work work" "Zug zug" } pop print flush ] SIGINFO add-signal-handler
IN: scratchpad load: 0.60  cmd: factor 41541 running 0.28u 0.16s
Zug zug
load: 0.71  cmd: factor 41541 running 0.28u 0.16s
Work work
load: 0.71  cmd: factor 41541 running 0.28u 0.16s
Me busy, leave me alone!
load: 0.73  cmd: factor 41541 running 0.28u 0.16s
Me not that kind of orc!