Add CLI for lcp-auto-subint

In preparation of another feature 'netlink-auto-subint', rename
lcp_main's field to "lcp_auto_subint".

Add CLI .short_help = "lcp lcp-auto-subint [on|enable|off|disable]"

Show status of the field on "lcp show" output.

lcpng.h

@@ -24,7 +24,7 @@ typedef struct lcp_main_s
   u16 msg_id_base;		    /* API message ID base */
   u8 default_namespace[LCP_NS_LEN]; /* default namespace if set */
   int default_ns_fd;
-  u8 auto_subint; /* Automatically create/delete LCP sub-interfaces */
+  u8 lcp_auto_subint; /* Automatically create/delete LCP sub-interfaces */
   /* Set when Unit testing */
   u8 test_mode;
 } lcp_main_t;