Fix not handling PGHOST environment for Unix domain socket connections (cont.)

 The previous commit is not good against PostgreSQL 9.3, because UnixSocketDir
is discarded and Unix_socket_directories comes to be used. In addition, parallel
writer processes uses both PGHOST and UnixSocketDir separately in the current
implementation. Thus, it seems to be better to use PGHOST only.
Support PostgreSQL 9.4beta1.
Update regression tests and documents for pg_bulkload 3.1.6.