loader: Add dependency fields to module structures.

* Declare 'requires' and 'enhances' text fields on module info structure.
* Rename 'nonoptreq' to 'optional_modules'.
* Update doxygen comments.

Still need to investigate dependencies among modules I cannot compile.

Change-Id: I3ad9547a0a6442409ff4e352a6d897bef2cc04bf

res/res_pjsip_dialog_info_body_generator.c

@@ -20,7 +20,6 @@
 	<depend>pjproject</depend>
 	<depend>res_pjsip</depend>
 	<depend>res_pjsip_pubsub</depend>
-	<depend>res_pjsip_exten_state</depend>
 	<support_level>core</support_level>
  ***/
 
@@ -219,4 +218,5 @@ AST_MODULE_INFO(ASTERISK_GPL_KEY, AST_MODFLAG_LOAD_ORDER, "PJSIP Extension State
 	.load = load_module,
 	.unload = unload_module,
 	.load_pri = AST_MODPRI_CHANNEL_DEPEND,
+	.requires = "res_pjsip,res_pjsip_pubsub",
 );