Use a system-supplied maximum number of signals on Mac OS X.

On Mac OS X the maximum number of signals is 31, with SIGUSR2 being number 31.  Due to the loop in _adjust_sigactions_ only going up to 30, SIGUSR1 and SIGUSR2 cannot be caught and processed by libqb.

 This gets change gets SIGUSR1 (#30) and SIGUSR2 (#31) working for my application.

Note that I'm using the presence of a Darwin specific define (__DARWIN_NSIG), which is always present on Darwin.  As far as I know, the more general NSIG macro should be available if _POSIX_C_SOURCE (any version) is defined when libqb is compiled.  This would address this issue in a completely portable fashion.