cache: document negative ttls and add about ttl

We've long supported negative ttls, for infinite cache, except the
documentation incorrectly showed one of our defaults as being 5 and not
-1. As well, with a negative ttl, we were actually making the HTTP
expired header go backwards. This changes it to go ahead ten years
instead.

Further, we add an cache-about-ttl option to set a different ttl for
about pages, which are now increasingly being filtered through markdown
or just sent statically anyway.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

cgit.h

@@ -200,6 +200,7 @@ struct cgit_config {
 	int cache_root_ttl;
 	int cache_scanrc_ttl;
 	int cache_static_ttl;
+	int cache_about_ttl;
 	int case_sensitive_sort;
 	int embedded;
 	int enable_filter_overrides;