r23667: Prevent storing of forbidden parameter names in registry

r23667: Prevent storing of forbidden parameter names in registry
configuration as values. I would really like to check whether
the valuename is a valid parameter name (with lp_parameter_is_valid)
here, but unfortunately, regedit cereates new values as
"New Value #1" (and so on) first, before dropping into the
rename box. So this is impossible here.

Michael