Merge branch 'sock-netns-with-tests' of https://github.com/masatake/util-linux

Merge branch 'sock-netns-with-tests' of https://github.com/masatake/util-linux

* 'sock-netns-with-tests' of https://github.com/masatake/util-linux:
  tests: (lsfd) verify SOCK.NETID and ENDPOINTS for sockets made in another netns
  tests: (lsns) verify the code finding an isolated netns via socket
  tests: (nsenter) verify the code entering the network ns via socket made in the ns
  tests: (test_sysinfo) add a helper to detect NS_GET_USERNS
  tests: (test_mkfds::foreign-sockets) new factory
  tests: (test_mkfds, refactor) use xmemdup newly added in xalloc.h
  xalloc.h: add xmemdup
  tests: (test_mkfds) fix a typo in an option name
  test_mkfds: (cosmetic) remove whitespaces between a function and its arguments