MFC r318175,r318178,r318179:

MFC r318175,r318178,r318179:

r318175:

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.

r318178:

procstat(1): document all possible `PRO` (network protocol) values

Reference the appropriate section 4 manpages for networking
protocols.

r318179:

procstat(1): also reference icmp(4) and sctp(4)

This was missed in the previous commit by accident.

MFC with:	r318178