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_1_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/1b384aff158a840fed4e9f4f366ba8d7c6ed42fb

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