Add Charset WIN1252 support.

src/backend/utils/mb/Unicode/utf8_to_win1252.map

src/backend/utils/mb/Unicode/win1252_to_utf8.map

...ckend/utils/mb/conversion_procs/utf8_and_win1252/Makefile

...s/mb/conversion_procs/utf8_and_win1252/utf8_and_win1252.c

src/backend/utils/mb/encnames.c

src/include/mb/pg_wchar.h