Function entry support

Now Mathomatic can accept any function strings, like f(x+y), by typing "set special_variable_characters ()" at the beginning of a session. It copies the entire function string into a variable name, so no operations can be performed on it, but it can exist and be displayed in any expression where everything else works. Functions can then perhaps be taken care of by another program if needed.

This is just added as a temporary measure, until I get better and add more function support to Mathomatic. Otherwise any function entry gives the error: "Named functions currently not implemented, except when using m4."
Trig and many other standard functions are handled with m4 Mathomatic (the program named rmath), which is usually included in any recent Mathomatic package.