Implement FMOD function for fixed-point modulo

Fixes #1021

man/rgbasm.5

@@ -319,6 +319,7 @@ delim $$
 .It Sy Name Ta Sy Operation
 .It Fn DIV x y Ta $x \[di] y$
 .It Fn MUL x y Ta $x \[mu] y$
+.It Fn FMOD x y Ta $x % y$
 .It Fn POW x y Ta $x$ to the $y$ power
 .It Fn LOG x y Ta Logarithm of $x$ to the base $y$
 .It Fn ROUND x Ta Round $x$ to the nearest integer