README: mention kernel requirement for ambient capabilities

Closes #23231.

README

@@ -31,6 +31,7 @@ LICENSE:
 
 REQUIREMENTS:
         Linux kernel ≥ 3.15
+                     ≥ 4.3 for ambient capabilities
                      ≥ 4.5 for pids controller in cgroup v2
                      ≥ 4.6 for cgroup namespaces
                      ≥ 4.9 for RENAME_NOREPLACE support in vfat

src/resolve/test-resolved-stream.c

@@ -110,7 +110,7 @@ static void *tcp_dns_server(void *p) {
 
         assert_se((bindfd = socket(AF_INET, SOCK_STREAM | SOCK_CLOEXEC, 0)) >= 0);
         assert_se(setsockopt(bindfd, SOL_SOCKET, SO_REUSEADDR, &(int){1}, sizeof(int)) >= 0);
-        assert_se(bind(bindfd, &SERVER_ADDRESS, sizeof(SERVER_ADDRESS)) >= 0);
+        assert_se(bind(bindfd, (struct sockaddr*)&SERVER_ADDRESS, sizeof(SERVER_ADDRESS)) >= 0);
         assert_se(listen(bindfd, 1) >= 0);
         assert_se((acceptfd = accept(bindfd, NULL, NULL)) >= 0);
         server_handle(acceptfd);
@@ -247,7 +247,7 @@ static void test_dns_stream(bool tls) {
         assert_se((clientfd = socket(AF_INET, SOCK_STREAM | SOCK_CLOEXEC, 0)) >= 0);
 
         for (int i = 0; i < 100; i++) {
-                r = connect(clientfd, &SERVER_ADDRESS, sizeof(SERVER_ADDRESS));
+                r = connect(clientfd, (struct sockaddr*)&SERVER_ADDRESS, sizeof(SERVER_ADDRESS));
                 if (r >= 0)
                         break;
                 usleep(EVENT_TIMEOUT_USEC / 100);

src/userdb/userdbctl.c

@@ -431,7 +431,7 @@ static int display_user(int argc, char *argv[], void *userdata) {
         if (table) {
                 _cleanup_free_ UidRange *uid_range = NULL;
                 int boundary_lines, uid_map_lines;
-                size_t n_uid_range;
+                size_t n_uid_range = 0;
 
                 r = uid_range_load_userns(&uid_range, &n_uid_range, "/proc/self/uid_map");
                 if (r < 0)
@@ -740,7 +740,7 @@ static int display_group(int argc, char *argv[], void *userdata) {
         if (table) {
                 _cleanup_free_ UidRange *gid_range = NULL;
                 int boundary_lines, gid_map_lines;
-                size_t n_gid_range;
+                size_t n_gid_range = 0;
 
                 r = uid_range_load_userns(&gid_range, &n_gid_range, "/proc/self/gid_map");
                 if (r < 0)