partial revert of commit d54dd8160b88709f3d243e1410a781e5de7fc187. We don't really need an abstraction over "printf-like" things. I can just use plain old FILEs. open_memstream (which conforms to POSIX.1) is what I should have used from the beginning in my mcache implementation.

the serialize "method" will soon be used in other context where using evbuffers would be awkward. Introduce a small abstraction over it: a printfn (print function) fprintf-like.

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

- don't expose the evbuffer where we store the cached page - cache to/from tabs instead of buffers

see github issue #1