Add support for other signals to support FreeBSD

[distefam]

FreeBSD does not have SIGRTMIN or SIGRTMAX support. Suggestion is to use SIGUSR{1,2}. While this is more limited than the current solution of using SIRRTMIN+n as it does not support 32 signals but only two it would be nice to be able to refresh i3blocks via a signal on FreeBSD. c.f. #23

[jacobsebek]

FreeBSD does have SIGRTMIN support, it is just not very well documented.

$ fgrep -r SIGRTMIN /usr/include
/usr/include/sys/signal.h:#define       SIGRTMIN        65

Therefore you can use realtime signals like

pkill -$((65 + 10)) i3blocks

To send a signal to the block with signal=10