tree-wide: add a single version of "static const int one = 1"

All over the place we define local variables for the various sockopts
that take a bool-like "int" value. Sometimes they are const, sometimes
static, sometimes both, sometimes neither.

Let's clean this up, introduce a common const variable "const_int_one"
(as well as one matching "const_int_zero") and use it everywhere, all
acorss the codebase.

src/basic/util.h

@@ -232,3 +232,6 @@ 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;