pgsql: Clean up code in libpq that obtains user's home directory: make a

Log Message:
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Clean up code in libpq that obtains user's home directory: make a single
subroutine that can hide platform dependencies. The WIN32 path is still
a stub, but I await a fix from one of the win32 hackers.
Also clean up unnecessary #ifdef WIN32 ugliness in a couple of places.

Modified Files:
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pgsql/src/interfaces/libpq:
fe-auth.c (r1.97 -> r1.98)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/fe-auth.c.diff?r1=1.97&r2=1.98)
fe-connect.c (r1.294 -> r1.295)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/fe-connect.c.diff?r1=1.294&r2=1.295)
fe-secure.c (r1.61 -> r1.62)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/fe-secure.c.diff?r1=1.61&r2=1.62)