Isolation: added option to disable tmpfs mount.

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

  {
      "isolation": {
          "automount": {
              "tmpfs": false
          }
      }
  }

src/nxt_process.h

@@ -75,6 +75,7 @@ typedef struct {
 
 typedef struct {
     uint8_t             language_deps;      /* 1-bit */
+    uint8_t             tmpfs;              /* 1-bit */
 } nxt_process_automount_t;