This patch introduces cmsg.go [1], which offers helpers to send and
receive fd through unix domain socket.
cmsg.go is copied from opencontainers/runc project against commit
4e7aeff610e63e4bef1ab60e17e39e07de5de1b8 ("libcontainer/utils: introduce
SendFds").
Links:
[1] https://github.com/opencontainers/runc/blob/master/libcontainer/utils/cmsg.go
Signed-off-by: Sheng Yong <shengyong2021@gmail.com>