lsfd: fill ENDPOINTS column for UNIX one-way sockets

lsfd: fill ENDPOINTS column for UNIX one-way sockets

With recvfrom(2) and sendto(2), we can use a UNIX datagram socket at
server side without assigning a peer address with connect(2). Here, I
call such sockets one-way sockets.

    # ss -x -p
    Netid State Recv-Q Send-Q        Local Address:Port  Peer Address:Port Process
    ...
    u_dgr ESTAB      0      0 /run/systemd/notify 13465               * 0  users:(("systemd",pid=1,fd=227))
    u_dgr ESTAB      0      0                   * 35438           * 13465  users:(("systemd-journal",pid=1280,fd=15))
    u_dgr ESTAB      0      0                   * 74792           * 13465  users:(("bluetoothd",pid=13874,fd=4))
    ...

The orignal code could not fill ENDPOINTS for UNIX one-way sockets:

  # ./original-lsfd -Q 'INODE == 13465' -oCOMMAND,PID,TYPE,INODE,NAME,ENDPOINTS
  COMMAND PID TYPE INODE NAME                                                ENDPOINTS
  systemd   1 UNIX 13465 state=connected path=/run/systemd/notify type=dgram

With this change, lsfd can fill the column:

  # ./new-lsfd -Q 'INODE == 13465' -oCOMMAND,PID,TYPE,INODE,NAME,ENDPOINTS
  COMMAND PID TYPE INODE NAME                                                ENDPOINTS
  systemd   1 UNIX 13465 state=connected path=/run/systemd/notify type=dgram 1280,systemd-journal,15rw
                                                                             13874,bluetoothd,4rw

Signed-off-by: Masatake YAMATO <yamato@redhat.com>