Ignore EINTR in workers when terminating due to SIGTERM

If the child process is killed using SIGTERM by the parent, send(2)
can fail with EINTR. Install a signal handler for SIGTERM and ignore
EINTR in this particular case.

Fixes #325.

Signed-off-by: Justus Winter <4winter@informatik.uni-hamburg.de>
Merge branch '0.3.3-fix-325'