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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REL9_0_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/996b21cbf24d0ff2ee16c1043fe443ae623474ea

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