Fix for Win32 pgport and pg_dumpall

The applied patch fixes libpgport backend linking on Win32. The
solution was to create palloc/pstrdup stubs in the backend and call them
from dirmod.c. Seems the DLLIMPORT modifier on CurrentMemoryContext
means it can't also be referenced in pgport.

This also fixes a broken pg_dumpall implementation on Win32.

All help via Claudio.

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  Bruce Momjian                        |  http://candle.pha.pa.us
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