add systemd support

This patch adds .service files and tmpfiles.d.

Signed-off-by: Paul J Stevens <paul@nfg.nl>
add --sieve switch for mbox import

Signed-off-by: Paul J Stevens <paul@nfg.nl>
systemd: add generated files
handle regression after coverity fixes
IMAP: avoid gmimestream in send_data

because filtered gmimestreams don't support SEEK_* :-(
lock the selfpipe to avoid spin-lock