wmaker: replaced macro by an inline function, in X Modifier initialisation

wmaker: replaced macro by an inline function, in X Modifier initialisation

A macro can be a source of problems, because the compiler has no type on
the arguments to make checks. Using an inline function allows to do those
checks, meaning clearer error messages, it provides clear info in case of
name collision, it is easier to maintain (no need for the hacky '\' for
multi-lines) and the scope of visibility can be controlled more easily (no
need for #undef).

Took opportunity to change a 0 to the constant NoSymbol which is the name
defined by X for this case and another to NULL which is the right way to
set a null pointer in C.

Signed-off-by: Christophe CURIS <christophe.curis@free.fr>