tests: (lsfd) make DGRAM socketpair to mitigate the change of protoname

tests: (lsfd) make DGRAM socketpair to mitigate the change of protoname

The protoname for a socket can be get from its xattr slot of
/proc/$PID/fd/$FD.

The protoname for (AF_UNIX, SOCK_STREAM) sockets was changed
from "UNIX" to "UNIX-STREAM" in
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=94531cfcbe79c3598acf96806627b2137ca32eb9

As the result, the test case, mkfds-socketpair doesn't work well on a
newer kernel.

To mitigate the impact of the change in the kernel, use (AF_UNIX, SOCK_DGRAM)
for making a socket pair.

In the commit, the protoname of (AF_UNIX, SOCK_DGRAM) sockets was also
changed to "UNIX-DGRAM". However, it was renamed back to "UNIX" in
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=0edf0824e0dc359ed76bf96af986e6570ca2c0b9

Partially close #1511.

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