- 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/Makefile.in

@@ -128,6 +128,7 @@ clean:
 	rm -f lex.clixon_xpath_parse.c
 	rm -f lex.clixon_api_path_parse.c
 	rm -f lex.clixon_instance_id_parse.c
+	rm -f *.gcda *.gcno *.gcov # coverage
 
 #############################################################################
 # Implicit rules for lex and yacc.