libpq error message refactoring, part 2

libpq error message refactoring

mingw: Define PGDLLEXPORT as __declspec (dllexport) as done for msvc

Fix PL/Perl build on Cygwin

Remove further unwanted linker flags from perl_embed_ldflags

Convert *GetDatum() and DatumGet*() macros to inline functions

Fix incorrect uses of Datum conversion macros

postgres_fdw: Remove useless DO block in test

Adjust assorted hint messages that list all valid options.

Add more detail why repalloc and pfree do not accept NULL pointers

Remove unneeded null pointer checks before PQfreemem()

Remove unnecessary casts in free() and pfree()

Avoid casting away const in sepgsql's quote_object_name.