pgsql: Centralize libpq's low-level code for dropping a connection.

Centralize libpq's low-level code for dropping a connection.

Create an internal function pqDropConnection that does the physical socket
close and cleans up closely-associated state. This removes a bunch of ad
hoc, not always consistent closure code. The ulterior motive is to have a
single place to wait for a spawned child backend to exit, but this seems
like good cleanup even if that never happens.

I went back and forth on whether to include "conn->status = CONNECTION_BAD"
in pqDropConnection's actions, but for the moment decided not to. Only a
minority of the call sites actually want that, and in any case it's
arguable that conn->status is slightly higher-level state, and thus not
part of this function's purview.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/210eb9b743c0645df05e5c8be4490ba4f09fc871

Modified Files
--------------
src/interfaces/libpq/fe-connect.c | 95 +++++++++++++----------------------
src/interfaces/libpq/fe-misc.c | 5 +--
src/interfaces/libpq/fe-protocol3.c | 4 +-
src/interfaces/libpq/libpq-int.h | 1 +
4 files changed, 38 insertions(+), 67 deletions(-)