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>