lsfd: fill ENDPOINTS column of unix socket using UNIX_DIAG_PEER information

lsfd: fill ENDPOINTS column of unix socket using UNIX_DIAG_PEER information

An example output:

   # ./lsfd -Q '((PID == 1) || (PID == 1896)) && (TYPE == "UNIX-STREAM") && ((FD == 1) || (FD == 278))' -o+ENDPOINTS
   COMMAND        PID USER ASSOC  XMODE        TYPE SOURCE MNTID INODE NAME                                             ENDPOINTS
   systemd          1 root   278 rw---m UNIX-STREAM sockfs     9 37012 state=connected path=/run/systemd/journal/stdout 1896,ModemManager,1
   ModemManager  1896 root     1 rw---- UNIX-STREAM sockfs     9 43126 state=connected                                  1,systemd,278
                                                                                                                        1870458,systemd-journal,68

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