fix for pg_regress.sh on mingw

With the attached patch "make check" runs cleanly for me under MSys/Mingw.

Description of changes:

. only use the -W flag on pwd for $pkglibdir. All the other paths need
to be seen as MSys type paths, whereas $pkglibdir needs to be expressed
as a genuine windows path.
. run single tests in the background and explicitly wait for them -
solves the problem of the MSys shell not waiting properly for the copy
test to finish.
. use pg_ctl to shut down the test postmaster - no more use of ad hoc
kill programs or the task manager.

With these changes there should no longer be any need to run regression
tests from cygwin for native Windows builds - you can simply run "make
check" from the build directory in the build environment.

cheers

andrew