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_interface.h

@@ -164,6 +164,12 @@ lcp_itf_pair_find_by_host (u32 host_sw_if_index)
   return (lip_db_by_host[host_sw_if_index]);
 }
 
+/**
+ * sub-interface auto creation/deletion for LCP
+ */
+void lcp_set_auto_subint (u8 is_auto);
+int lcp_auto_subint (void);
+
 typedef void (*lcp_itf_pair_add_cb_t) (lcp_itf_pair_t *);
 typedef void (*lcp_itf_pair_del_cb_t) (lcp_itf_pair_t *);