Add automatic LCP creation

Update the pair_config() parser to follow suite.

When the configuration 'lcp-auto-subint' is set, and the interface at
hand is a subinterface, in lcp_itf_interface_add_del():

- if it's a deletion and we're a sub-int, and we have a LIP: delete it.
- if it's a creation and we're a sub-int, and our parent has a LIP, create one.

Fix a few logging consistency issues (pair_del), and in
pair_delete_by_index() ensure that the right namespace is selected.

Due to this quirk with lip->lip_namespace not wanting to be a vec_dup()
string, rewrite them all to be strdup/free instead.

lcpng.h

@@ -24,6 +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 */
   /* Set when Unit testing */
   u8 test_mode;
 } lcp_main_t;