Don't create [general] voicemail context when using users.conf

Prior to this patch, app_voicemail would create a [general] context when parsing users.conf.

(closes issue #18891)
Reported by: pdugas
Patches: 
      app_voicemail-ignore-general.patch uploaded by pdugas (license 1222)
      app_voicemail-ignore-general-style-guidelines.patch uploaded by seanbright (license 71)
Tested by: pdugas

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@319367 65c4cc65-6c06-0410-ace0-fbb531ad65f3

apps/app_voicemail.c

@@ -12140,6 +12140,9 @@ static int load_config(int reload)
 
 		if (ucfg) {	
 			for (cat = ast_category_browse(ucfg, NULL); cat ; cat = ast_category_browse(ucfg, cat)) {
+				if (!strcasecmp(cat, "general")) {
+					continue;
+				}
 				if (!ast_true(ast_config_option(ucfg, cat, "hasvoicemail")))
 					continue;
 				if ((current = find_or_create(userscontext, cat))) {