[RFC] use buffered stdio (lightly tested)

[Eric Wong]

In the future, I think it could be even more beneficial to
buffer to zlib (if the client accepts gzip) and bypass stdio
buffering in those cases.  Smaller responses via gzip means less
IPC overhead and wakeups for the reader.

cgit is linked against zlib anyways because of git, so no extra
linkage, either.  But we'll need uncompressed code paths anyways
for clients which don't support gzip.