libpq.dll on win32

This patch aims to correct the issues with libpq.dll exporting too much
on win32. Contains the following changes:

* Exports PQgetssl() even if SSL support is not compiled in. In this
case, it always returns NULL. This is required so programs that expect
SSL version of the library does not crash if they don't get it.

* Create a global flag in Makefile.shlib named "DLL_DEFNAME" used to
specify a .def file when generating a DLL. If not specified, falls back
to the old behaviour of exporting everything in the file.

* Adds required functions to libpqdll.def and libpqddll.def. This
includes the functions not in the libpq API that are used by initdb and
imported from libpq.dll (per recent mail discussion)

//Magnus