fix: use && instead of &

This is to silence the sparse `warning: dubious usage of x & !y`.
The difference is insignificant and I am not going to argue sparse here.

src/io/conf.c

@@ -17,7 +17,7 @@
 static const char *strmat(const char *s1, const char *s2) PURE NONNULL((1, 2));
 static const char *strmat(const char *s1, const char *s2)
 {
-	while ((*s1 == *s2) & !!*s1)
+	while ((*s1 == *s2) && *s1)
 		s1++, s2++;
 	return s1;
 }