proposal: net: UnixListener: provide a way to get the `unlink` attribute

[frankli0324]

#11826 prevents listeners not created by the process to be unlinked.
#13877 allows devs to explicitly specify whether go should unlink a listener

why does go design a setter SetUnlinkOnClose but not a getter? was there any concerns?
would golang provide a getter?

[ianlancetaylor]

Are there cases where a getter is useful? We didn't provide one because it's hard to see a use case.

[frankli0324]

it's actually a very specific case, but I felt it could be generalized...
I came along with some lines of code which enumerates all file descriptors, and SetUnlinkOnClose(false) if listener.(UnixListener). it passes these file descriptors to a subprocess created by StartProcess. by doing this, it achieves graceful restart. on startup, the code SetUnlinkOnClose(true) on all the listeners passed to it, removing all the file descriptors the parent passed to the child.
however, the common sense is that a listener should not be unlinked if it's not created by the process. so the parent process have to tell the child which fds are to be preserved by passing a list.
without the getter, the parent process have to maintain a list containing all the fds to be preserved upon creating the listeners, this adds complexity, concurrency locks (listeners are created asynchronously) and of course an extra global variable. but with a getter, the parent process only have to build that list when forking the child process.