test: (test_mkfds) add -O option for describing output values

test: (test_mkfds) add -O option for describing output values

Some factories prints some extra values in addition to the pid of
the process opening file descriptors.

Unlike parameters, test_mkfds didn't provide the way to explain
the extra values. The new option --output-values/-O provies it.

Example output:

  $ ./test_mkfds -O eventfd
  NTH DESCRIPTION
  0 the pid owning the file descriptor(s)
  1 the pid of child process

  $ ./test_mkfds -O cdev-tun
  NTH DESCRIPTION
  0 the pid owning the file descriptor(s)
  1 the network device name

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