Modernize the initalization of sigproptbl.

Use C99 designators to set the value of each slot and the nitems macro to
check for valid entries. In the process, switch to indexing by signal
number rather than signal-1 for improved clarity.

Obtained from: CheriBSD (a6053c5abf03a5f53bbfcdd3a26429383f67e09f)
Sponsored by: DARPA, AFRL
Reviewed by: kib