util: improve signal_to_string, signal_from_string

util: improve signal_to_string, signal_from_string

signal_to_string:
 Produce names for SIGRTMIN+n.
 Never give an "n/a" result. In the worst case give the number itself as
 a string.

signal_from_string:
 Parse "RTMIN+n".
 Parse any valid signal number.