Renamed the functions working with "struct Function".

Renamed the functions working with "struct Function".
Fixed a problem with K&R functions: In a function with no return type
specified, the compiler did not allow a simple "return" statement. However,
there was no "void" type at that time, so it was not possible to specify
something else. The solution is to allow omission of a return value in a
K&R function with an implicit int type. Other types or an explicit int is
still checked.

git-svn-id: svn://svn.cc65.org/cc65/trunk@1302 b7a2c559-68d2-44c3-8de9-860c34a00d81