Isolation: added option to disable "procfs" mount.

Now users can disable the default procfs mount point
in the rootfs.

 {
     "isolation": {
         "automount": {
             "procfs": false
         }
     }
 }

src/nxt_conf_validation.c

@@ -844,6 +844,9 @@ static nxt_conf_vldt_object_t  nxt_conf_vldt_app_automount_members[] = {
     }, {
         .name       = nxt_string("tmpfs"),
         .type       = NXT_CONF_VLDT_BOOLEAN,
+    }, {
+        .name       = nxt_string("procfs"),
+        .type       = NXT_CONF_VLDT_BOOLEAN,
     },
 
     NXT_CONF_VLDT_END