Separate out the string parsing for .include/.library file names and macro arguments.

They behave observably different from generic string parsing and trying
to account for them generically just gets in the way.
.rept is treated the same as a macro.

parse.h

@@ -26,6 +26,9 @@ SYMBOL         *get_op(
 char           *getstring(
     char *cp,
     char **endp);
+char           *getstring_fn(
+    char *cp,
+    char **endp);
 char           *get_symbol(
     char *cp,
     char **endp,