patch for subs_algebraic

Hello, I discovered that the subs_algebraic subs_option does not yet go into subexpressions. The attached patch makes this happen. Furthermore nummatches was made into an int (it was an unsigned) because the unsigned was not used anyway and the static_cast to integer that was introduced in version 1.66.2.13 of mul.cpp completely kills subs_algebraic. I.e., it converts the maximum unsigned integer to -1, which is not good. Greetings, Chris Dams