procstat(1): clarify the Signal Disposition section

• Fix a typo (SIGIGN -> SIG_IGN). Use .Dv when referencing SIG_IGN.
• Use semi-colons as soft breaks when separating sentences for the FLAGS section.
• Tweak wording for C slightly to flow better and to be a bit more technically correct (signals with handlers installed will be caught by the target program).
• Reference signal(3) in the SEE ALSO section.

MFC after: 3 weeks
Sponsored by: Dell EMC Isilon