beam: Make extracted Fds CloseOnExec

Grab forklock to make sure no forks accidentally inherit the new fds
before they are made CLOEXEC There is a slight race condition between
ReadMsgUnix returns and when we grap the lock, so this is not
perfect. Unfortunately There is no way to pass MSG_CMSG_CLOEXEC to
recvmsg() nor any way to implement non-blocking i/o in go, so this is
hard to fix.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Merge pull request #77 from alexlarsson/beam3-mini

beam: Make extracted Fds CloseOnExec