sideband: use writev(3p) to send pktlines

sideband: use writev(3p) to send pktlines

Every pktline that we send out via `send_sideband()` currently requires
two syscalls: one to write the pktline's length, and one to send its
data. This typically isn't all that much of a problem, but under extreme
load the syscalls may cause contention in the kernel.

Refactor the code to instead use the newly introduced writev(3p) infra
so that we can send out the data with a single syscall. This reduces the
number of syscalls from around 133,000 calls to write(3p) to around
67,000 calls to writev(3p).

Suggested-by: Jeff King <peff@peff.net>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>