lang/ratfor: fix build on platforms where char is unsigned

Turns out the code anticipates this problem and has support for using
"signed char" over "char" where "char" is unsigned by default.

Use "signed char" unconditionally to fix the port on all platforms.

While we are at it, pet portclippy.

PR:		258628, 283509
Approved by:	bofh
MFH:		2024Q4