Make snapshot feature configurable

Snapshots can now be enabled/disabled by default for all repositories in
cgitrc with param "snapshots". Additionally, any repo can override the
default setting with param "repo.snapshots".

By default, no snapshotting is enabled.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>

cgit.h

@@ -21,6 +21,7 @@ struct repoinfo {
 	char *path;
 	char *desc;
 	char *owner;
+	int snapshots;
 };
 
 struct repolist {
@@ -61,6 +62,7 @@ extern char *cgit_virtual_root;
 extern char *cgit_cache_root;
 
 extern int cgit_nocache;
+extern int cgit_snapshots;
 extern int cgit_max_lock_attempts;
 extern int cgit_cache_root_ttl;
 extern int cgit_cache_repo_ttl;