net: UnixListener auto unlinks socket files behavior should be changeable

[absolute8511]

The unlink flag should be designed as changeable from outside to avoid auto unlink unix domain socket file. It can be useful while implementing the graceful restart. The graceful restart need the new process listen on the same file and then close the old one. If the old one auto unlink the socket file, it will cause the socket listened by the new process is not available from the client.
The graceful restart can be found here https://github.com/facebookgo/grace

[danp]

Should this be accomplished by making UnixListener.unlink public?

[absolute8511]

Maybe an option for auto unlink and a public method would be great. (just like FD_CLOEXEC)

[rsc]

Too late for new API.

[quentinmit]

See also #17131

[gopherbot]

CL https://golang.org/cl/32099 mentions this issue.