delete pages that are older than one hour, and scan the cache only once every five minutes. The previous defaults were a bit too much conservative.

instead of keeping the struct mcache that's private to mcache.c and used only once, just define its fields as (static) variables. While here, mark as static also gemtext_prefixes.

it's not spelled `util.h' because otherwise it'd get included instead of /usr/include/util.h