unix: escape all null bytes in abstract unix domain socket

Abstract unix domain socket may embed null characters,
these should be translated to '@' when printed out to
proc the same way the null prefix is currently being
translated.

This helps for tools such as netstat, lsof and the proc
based implementation in ss to show all the significant
bytes of the name (instead of getting cut at the first
null occurrence).

Signed-off-by: Isaac Boukris <iboukris@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>