CA-139357: ignore SIGCHLD in master tapback

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
Reviewed-by: Frediano Ziglio <frediano.ziglio@citrix.com>
CA-136792: produce blkback-style stats

Produce I/O statistics in /dev/shm/vbd3-domid-devid. We use one file for
all seven counters as otherwise we're wasting too much memory. Stats are
updated once every thirty seconds to minimise impact on performance.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
CA-136792: produce read/write latency stats

Produce read/write latency stats in /dev/shm/vbd3-<domid>-<devid>/statistics
along with the other stats, in order to avoid using a separate file as
this consumes additional memory and we're already under heavy pressure to
minimise tapdisk's memory footprint.

Also, produce stats every five seconds as this is the same frequency the
RRD script reads this file.

In blkback, rd_usecs/wr_usecs stats are cleared each time the sysfs file
is read. In tapdisk it's impossible to tell when the file got read,
therefore we reset these stats right after updating /dev/shm.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>