Add support for Win1252 encoding.

doc/src/sgml/charset.sgml

doc/src/sgml/func.sgml

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

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

src/backend/utils/mb/conversion_procs/Makefile

...s/mb/conversion_procs/utf8_and_iso8859/utf8_and_iso8859.c

...s/mb/conversion_procs/utf8_and_win1250/utf8_and_win1250.c

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

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

...s/mb/conversion_procs/utf8_and_win1256/utf8_and_win1256.c

...ils/mb/conversion_procs/utf8_and_win874/utf8_and_win874.c

src/backend/utils/mb/encnames.c

src/backend/utils/mb/wchar.c

src/bin/initdb/initdb.c

src/include/mb/pg_wchar.h

src/test/regress/expected/conversion.out

src/test/regress/sql/conversion.sql