Suppress signed-vs-unsigned-char warnings.

src/backend/catalog/pg_conversion.c

src/backend/commands/copy.c

src/backend/libpq/pqcomm.c

src/backend/libpq/pqformat.c

src/backend/postmaster/pgstat.c

src/backend/tcop/fastpath.c

src/backend/tcop/postgres.c

src/backend/utils/adt/ascii.c

src/backend/utils/adt/encode.c

src/backend/utils/adt/like.c

src/backend/utils/adt/like_match.c

src/backend/utils/adt/regexp.c

src/backend/utils/adt/selfuncs.c

src/backend/utils/adt/varbit.c

src/backend/utils/adt/varlena.c

src/backend/utils/mb/conv.c

...d/utils/mb/conversion_procs/ascii_and_mic/ascii_and_mic.c

...s/mb/conversion_procs/cyrillic_and_mic/cyrillic_and_mic.c

...utils/mb/conversion_procs/euc_cn_and_mic/euc_cn_and_mic.c

...ils/mb/conversion_procs/euc_jp_and_sjis/euc_jp_and_sjis.c