Find out the supported max number of signals on the platform

[vorner]

Sometimes it is nice to have an array indexed by signals preallocated. To do that, we would need to know what the max signal number on a given platform is.

This should be possible to find out by calling sigaction with both pointers set to NULL with increasing number as the signal. This assumes there are no „holes“ in the signal numbers, though ‒ is that true?