tree-wide: introduce new SOCKADDR_UN_LEN() macro, and use it everywhere

The macro determines the right length of a AF_UNIX "struct sockaddr_un" to pass to
connect() or bind(). It automatically figures out if the socket refers to an
abstract namespace socket, or a socket in the file system, and properly handles
the full length of the path field.

This macro is not only safer, but also simpler to use, than the usual
offsetof() + strlen() logic.

src/test/test-socket-util.c

@@ -343,6 +343,21 @@ static void test_sockaddr_equal(void) {
         assert_se(!sockaddr_equal(&b, &c));
 }
 
+static void test_sockaddr_un_len(void) {
+        static const struct sockaddr_un fs = {
+                .sun_family = AF_UNIX,
+                .sun_path = "/foo/bar/waldo",
+        };
+
+        static const struct sockaddr_un abstract = {
+                .sun_family = AF_UNIX,
+                .sun_path = "\0foobar",
+        };
+
+        assert_se(SOCKADDR_UN_LEN(fs) == offsetof(struct sockaddr_un, sun_path) + strlen(fs.sun_path));
+        assert_se(SOCKADDR_UN_LEN(abstract) == offsetof(struct sockaddr_un, sun_path) + 1 + strlen(abstract.sun_path + 1));
+}
+
 int main(int argc, char *argv[]) {
 
         log_set_max_level(LOG_DEBUG);
@@ -363,5 +378,7 @@ int main(int argc, char *argv[]) {
 
         test_sockaddr_equal();
 
+        test_sockaddr_un_len();
+
         return 0;
 }