tests: (lsfd::option-inet) get child-processes' pids via fifo

tests: (lsfd::option-inet) get child-processes' pids via fifo

Close #2399.

The original code assumes the background processes prepare sockets enough
fast. #2399 showed the assumption was wrong; lsfd reported half-cooked
sockets.

To avoid the timing issue, the test case with this change uses a fifo
to receive pids from the child test_mkfds processes. test_mkfds reports
its pid after cooking the sockets. When the option-inet script recives the pid,
we can expect the sockets are ready.

Note: bash's coproc cannot be used here; it supports only one co-process at
once.

Analysed-by: Alexander Kanavin <alex.kanavin@gmail.com>
Signed-off-by: Masatake YAMATO <yamato@redhat.com>