Add caching infrastructure

This enables internal caching of page output.

Page requests are split into four groups:
  1) repo listing (front page)
  2) repo summary
  3) repo pages w/symbolic references in query string
  4) repo pages w/constant sha1's in query string

Each group has a TTL specified in minutes. When a page is requested, a cached
filename is stat(2)'ed and st_mtime is compared to time(2). If TTL has expired
(or the file didn't exist), the cached file is regenerated.

When generating a cached file, locking is used to avoid parallell processing
of the request. If multiple processes tries to aquire the same lock, the ones
who fail to get the lock serves the (expired) cached file. If the cached file
don't exist, the process instead calls sched_yield(2) before restarting the
request processing.

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

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