CA-120823: Don't use a tmp file for logging and handle SIGHUP

tapdisk now handles the SIGHUP signal, closing and opening the log file
in order to allow logrotate to rotate the log file.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
Reviewed-by: Germano Percossi <germano.percossi@citrix.com>

GitHub: closes #61 on xapi-project/blktap