lsfd: re-fill unix socket paths with sockdiag netlink interface

lsfd: re-fill unix socket paths with sockdiag netlink interface

This commit is mostly based on Thomas Weißschuh's work,
https://github.com/t-8ch/util-linux/commit/06030390a5e6a16cc4b914bbf5fcedd3b6d83608.

Unlike the original work, this commit keeps /proc related code.

Unlike /proc interface, the sockdiag information source doesn't provide
enough information for filling struct unix_xinfo::st member. So this
commit uses /proc interface for filling the most of all unix_xinfo
members as before.

On the other hande, as discussed in
https://github.com/util-linux/util-linux/pull/2067, the /proc
interface in unreliable if a unix path name includes newline
characters. This commit uses the sockdiag interface to compensate
for the weakness of the /proc interface.