- Restructure plugin module using a "module struct" rather than global variables.

This include plugin module init and exit functions
  New type: clixon_plugin_t exposed via public API while struct clixon_plugin is private
- Makefile changes for static linkage using "LINKAGE=static"
- Moved nacm external init to later stage in backend_main

lib/src/clixon_yang_internal.h

@@ -60,6 +60,8 @@ struct yang_type_cache{
 typedef struct yang_type_cache yang_type_cache;
 
 /*! yang statement 
+ * This is an internal type, not exposed in the API
+ * The external type is "yang_stmt" defined in clixon_yang.h
  */
 struct yang_stmt{
     int                ys_len;       /* Number of children */