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

(cherry picked from commit 147aeb1ee48f8aeb8e4892e3271a3231388a412c)