lldpd: whitelist sendto, poll, recvmsg and readv in seccomp sandbox

With privilege separation and seccomp enabled, the monitor thread was
dying due to seccomp errors, while leaving the child alive. This whitelists
some extra syscalls to allow the monitor process to stay alive.
Merge pull request #70 from chutz/seccomp-whitelist

seccomp: whitelist sendto, poll, recvmsg and readv in seccomp sandbox