Switch to exclusively using global ctx

Drop the context parameter from the following functions (and all static
helpers used by them) and use the global context instead:

* cgit_print_http_headers()
* cgit_print_docstart()
* cgit_print_pageheader()

Remove context parameter from all commands

Drop the context parameter from the following functions (and all static
helpers used by them) and use the global context instead:

* cgit_get_cmd()
* All cgit command functions.
* cgit_clone_info()
* cgit_clone_objects()
* cgit_clone_head()
* cgit_print_plain()
* cgit_show_stats()

In initialization routines, use the global context variable instead of
passing a pointer around locally.

Remove callback data parameter for cache slots

This is no longer needed since the context is always read from the
global context variable.

Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>

ui-repolist.c

@@ -259,9 +259,9 @@ void cgit_print_repolist()
 		++columns;
 
 	ctx.page.title = ctx.cfg.root_title;
-	cgit_print_http_headers(&ctx);
-	cgit_print_docstart(&ctx);
-	cgit_print_pageheader(&ctx);
+	cgit_print_http_headers();
+	cgit_print_docstart();
+	cgit_print_pageheader();
 
 	if (ctx.cfg.index_header)
 		html_include(ctx.cfg.index_header);