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_sdp_rtp.c

@@ -1970,4 +1970,5 @@ AST_MODULE_INFO(ASTERISK_GPL_KEY, AST_MODFLAG_LOAD_ORDER, "PJSIP SDP RTP/AVP str
 	.load = load_module,
 	.unload = unload_module,
 	.load_pri = AST_MODPRI_CHANNEL_DRIVER,
+	.requires = "res_pjsip,res_pjsip_session",
 );