Regression tests fails to start on system without unix sockets.

The regression tests will fail to start on a system that doesn't
have, or wasn't compiled for, unix domain sockets.

The pg_regress script will start the postmaster with the -i
option in case of QNX and BEOS, but not for instance in case of
WIN32.

I see some options to fix this:
- Always start with -i
- Make the "unix_sockets" variable depend on HAVE_UNIX_SOCKETS intead of listen the OSs.


Kurt