pgsql: Fix bugs in PQhost().

Fix bugs in PQhost().

In the platform that doesn't support Unix-domain socket, when
neither host nor hostaddr are specified, the default host
'localhost' is used to connect to the server and PQhost() must
return that, but it didn't. This patch fixes PQhost() so that
it returns the default host in that case.

Also this patch fixes PQhost() so that it doesn't return
Unix-domain socket directory path in the platform that doesn't
support Unix-domain socket.

Back-patch to all supported versions.

Branch
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REL8_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/5525529db191b6f4b893d445e5c801bc456ba3bb

Modified Files
--------------
src/interfaces/libpq/fe-connect.c | 11 ++++++++++-
1 file changed, 10 insertions(+), 1 deletion(-)