IPv6 cleanups

I have applied the attached patch to allow longer IPv6 addresss in libpq
'hostaddr' connection string. This doesn't appear to affect any
backward compatibility because only a pointer is used in the pgconn
structure.

I also updated a comment and some of the docs.

-- 
  Bruce Momjian                        |  http://candle.pha.pa.us
  pgman@candle.pha.pa.us               |  (610) 359-1001
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