pgsql: Fix usage of char2wchar/wchar2char.

Log Message:
-----------
Fix usage of char2wchar/wchar2char. Changes:
- pg_wchar and wchar_t could have different size, so char2wchar
doesn't call pg_mb2wchar_with_len to prevent out-of-bound
memory bug
- make char2wchar/wchar2char symmetric, now they should not be
called with C-locale because mbstowcs/wcstombs oftenly doesn't
work correct with C-locale.
- Text parser uses pg_mb2wchar_with_len directly in case of
C-locale and multibyte encoding

Per bug report by Hiroshi Inoue <inoue(at)tpf(dot)co(dot)jp> and
following discussion.

Backpatch up to 8.2 when multybyte support was implemented in tsearch.

Modified Files:
--------------
pgsql/src/backend/tsearch:
wparser_def.c (r1.20 -> r1.21)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/tsearch/wparser_def.c?r1=1.20&r2=1.21)
pgsql/src/backend/utils/mb:
mbutils.c (r1.78 -> r1.79)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/mbutils.c?r1=1.78&r2=1.79)