Zero-copy GET responses with splice()

This commit lets the object server use splice() and tee() to move data
from disk to the network without ever copying it into user space.

Requires the "splicetee" library and Linux. Sorry, FreeBSD folks. You
still have the old mechanism, as does anyone who doesn't want to
install splicetee. It's optional. (While I normally dislike optional
dependencies, this one requires a particular OS, so I'm okay with it.)

Note that this only applies to GET responses without Range headers. It
can easily be extended to single-range GET requests, but this commit
leaves that for future work. Same goes for PUT requests, or at least
non-chunked ones.

Change-Id: I1d107b777e6f0ced77a054fe5758401017d37db9