Fix building on OpenSolaris (without _NSIG definition).
Fix building on OpenSolaris (without _NSIG definition).

On OpenSolaris, the _NSIG macro is not defined, but NSIG is; as it's
not possible to use NSIG under glibc (it would miss the SIGRT*
series), define a SIGNAL_UPPER_BOUND depending on the system (and
error out if the system lacks both NSIG and _NSIG).

Distrotech/libdaemon,richardcochran/libdaemon,Distrotech/libdaemon,Distrotech/libdaemon,richardcochran/libdaemon