pgsql: Tweak libpq's PQhost, PQhostaddr, and psql's \connect

Tweak libpq's PQhost, PQhostaddr, and psql's \connect

Fixes some problems introduced by 6e5f8d489acc:

* When reusing conninfo data from the previous connection in \connect,
  the host address should only be reused if it was specified as
  hostaddr; if it wasn't, then 'host' is resolved afresh.  We were
  reusing the same IP address, which ignores a possible DNS change
  as well as any other addresses that the name resolves to than the
  one that was used in the original connection.

* PQhost, PQhostaddr: Don't present user-specified hostaddr when we have
  an inet_net_ntop-produced equivalent address.  The latter has been
  put in canonical format, which is cleaner (so it produces "127.0.0.1"
  when given "host=2130706433", for example).

* Document the hostaddr-reusing aspect of \connect.

* Fix some code comments

Author: Fabien Coelho
Reported-by: Noah Misch
Discussion: https://postgr.es/m/20190527203713.GA58392@gust.leadboat.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/313f56ce2d1b9dfd3483e4f39611baa27852835a

Modified Files
--------------
doc/src/sgml/ref/psql-ref.sgml    |  3 +++
src/bin/psql/command.c            | 41 ++++++++++++++++++++++++++++++---------
src/interfaces/libpq/fe-connect.c | 20 ++++++++-----------
3 files changed, 43 insertions(+), 21 deletions(-)