core: taint if /usr is unmerged

9afd5e7b97 introduced a build-time
taint, introduce a runtime one as well, in preparation for
removing support for unmerged-usr in a future release

src/core/manager.c

@@ -4377,12 +4377,16 @@ char* manager_taint_string(const Manager *m) {
 
         assert(m);
 
-        const char* stage[11] = {};
+        const char* stage[12] = {};
         size_t n = 0;
 
         if (m->taint_usr)
                 stage[n++] = "split-usr";
 
+        _cleanup_free_ char *usrbin = NULL;
+        if (readlink_malloc("/bin", &usrbin) < 0 || !PATH_IN_SET(usrbin, "usr/bin", "/usr/bin"))
+                stage[n++] = "unmerged-usr";
+
         if (access("/proc/cgroups", F_OK) < 0)
                 stage[n++] = "cgroups-missing";