Add a cache-snapshot-ttl configuration variable

This can be used to specify the TTL for snapshots. Snapshots are usually
static and do not ever change. On the other hand, tarball generation is
CPU intensive.

One use case of this setting (apart from increasing the lifetime of
snapshot cache slots) is caching of snapshots while disabling the cache
for static/dynamic HTML pages (by setting TTL to zero for everything
except for snapshot requests).

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

cgit.h

@@ -210,6 +210,7 @@ struct cgit_config {
 	int cache_scanrc_ttl;
 	int cache_static_ttl;
 	int cache_about_ttl;
+	int cache_snapshot_ttl;
 	int case_sensitive_sort;
 	int embedded;
 	int enable_filter_overrides;