sighandler_t compatibility issue with OpenBSD

[Nikos Mavrogiannopoulos]

On Sat, 2016-02-20 at 14:41 +0100, Björn Ketelaars wrote:
> Both OpenBSD and FreeBSD use sig_t instead of sighandler_t. This is 
> kind of an
> issue for src/occtl/unix.c. Fortunate for us src/common/system.h 
> (which is
> included in unix.c) takes great care of handling sighandler_t/sig_t. 
> Proposed

Thank you. Applied too.