util: drop const_int_one/const_int_zero again

There's only a single user remaining now that we have setsockopt_int(),
let's define those variables locally.

This more or less reverts 6d5e65f645.

src/basic/util.h

@@ -232,6 +232,3 @@ int version(void);
 int str_verscmp(const char *s1, const char *s2);
 
 void disable_coredumps(void);
-
-extern const int const_int_zero;
-extern const int const_int_one;