Fix 2001/cheong to work with clang

I had to make a new function (also four letter word to match the
formatting) that is allowed to have an int as the third arg to it. I
chose the word pain :-) as it's a pain that clang is this way without
providing any option to disable it.

Note that the arg must be an even number of digits. This threw me off
until I read the README.md because on different systems I got different
results for odd number of digits args despite the fact there should be
no difference here as the diff shows.

2001/cheong/cheong.c

@@ -1,12 +1,14 @@
 #include <stdio.h>
-int l;int main(int o,char **O,
+int l;int pain(int o,char **O,
 int I){char c,*D=O[1];if(o>0){
 for(l=0;D[l              ];D[l
 ++]-=10){D   [l++]-=120;D[l]-=
-110;while   (!main(0,O,l))D[l]
+110;while   (!pain(0,O,l))D[l]
 +=   20;   putchar((D[l]+1032)
 /20   )   ;}putchar(10);}else{
 c=o+     (D[I]+82)%10-(I>l/2)*
 (D[I-l+I]+72)/10-9;D[I]+=I<0?0
-:!(o=main(c/10,O,I-1))*((c+999
+:!(o=pain(c/10,O,I-1))*((c+999
 )%10-(D[I]+92)%10);}return o;}
+int main(int o,char **O)     {
+return pain(o, O, l);	     }