pgsql: Const-ify the arguments of str_tolower() and friends to suppress

Log Message:
-----------
Const-ify the arguments of str_tolower() and friends to suppress compile
warnings. Clean up various unneeded cruft that was left behind after
creating those routines. Introduce some convenience functions str_tolower_z
etc to eliminate tedious and error-prone double arguments in formatting.c.
(Currently there seems no need to export the latter, but maybe reconsider
this later.)

Modified Files:
--------------
pgsql/src/backend/utils/adt:
formatting.c (r1.144 -> r1.145)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/formatting.c?r1=1.144&r2=1.145)
oracle_compat.c (r1.81 -> r1.82)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/oracle_compat.c?r1=1.81&r2=1.82)
pgsql/src/include/utils:
formatting.h (r1.19 -> r1.20)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/formatting.h?r1=1.19&r2=1.20)