basic: Fix capability_ambient_set_apply for kernels < 4.3

systemd/systemd#14133 made
capability_ambient_set_apply() acquire capabilities that were explicitly
asked for and drop all others. This change means the function is called
even with an empty capability set, opening up a code path for users
without ambient capabilities to call this function. This function will
error with EINVAL out on kernels < 4.3 because PR_CAP_AMBIENT is not
understood. This turns capability_ambient_set_apply() into a noop for
kernels < 4.3

Fixes systemd/systemd#15225

(cherry picked from commit 7ea4392)

src/basic/capability-util.c

@@ -107,6 +107,10 @@ int capability_ambient_set_apply(uint64_t set, bool also_inherit) {
         unsigned long i;
         int r;
 
+        /* Check that we can use PR_CAP_AMBIENT or quit early. */
+        if (!ambient_capabilities_supported())
+                return 0;
+
         /* Add the capabilities to the ambient set. */
 
         if (also_inherit) {