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_process.h

@@ -76,6 +76,7 @@ typedef struct {
 typedef struct {
     uint8_t             language_deps;      /* 1-bit */
     uint8_t             tmpfs;              /* 1-bit */
+    uint8_t             procfs;             /* 1-bit */
 } nxt_process_automount_t;