pgsql: Move wchar2char() and char2wchar() from tsearch into /mb to be

Log Message:
-----------
Move wchar2char() and char2wchar() from tsearch into /mb to be easier to
use for other modules;  also move pnstrdup().

Clean up code slightly.

Modified Files:
--------------
    pgsql/src/backend/tsearch:
        ts_locale.c (r1.8 -> r1.9)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/tsearch/ts_locale.c?r1=1.8&r2=1.9)
        ts_utils.c (r1.9 -> r1.10)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/tsearch/ts_utils.c?r1=1.9&r2=1.10)
    pgsql/src/backend/utils/mb:
        mbutils.c (r1.71 -> r1.72)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/mbutils.c?r1=1.71&r2=1.72)
    pgsql/src/backend/utils/mmgr:
        mcxt.c (r1.63 -> r1.64)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mmgr/mcxt.c?r1=1.63&r2=1.64)
    pgsql/src/include/mb:
        pg_wchar.h (r1.78 -> r1.79)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/mb/pg_wchar.h?r1=1.78&r2=1.79)
    pgsql/src/include/tsearch:
        ts_locale.h (r1.6 -> r1.7)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/tsearch/ts_locale.h?r1=1.6&r2=1.7)
        ts_public.h (r1.9 -> r1.10)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/tsearch/ts_public.h?r1=1.9&r2=1.10)
    pgsql/src/include/utils:
        palloc.h (r1.38 -> r1.39)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/palloc.h?r1=1.38&r2=1.39)