tests: (test_mkfds) monitor stdin by default

tests: (test_mkfds) monitor stdin by default

After making file descriptors as specified in arguments, test_mkfds
waits for an event with pselect. When test_mkfds gets the event, it
exits.

The original code monitored stdin when the test_mkfds process ran in
the foreground. However, monitoring stdin is useful even when it runs
in the background. ts/lsfd/mkfds-* test cases run test_mkfds as a
co-process.  So the test cases can terminate their test_mkfds
processes with echo command if the processes monitor stdin.

This change also adds -X/--dont-monitor-stdin option. As the name
shows, with the option, test_mkfds doesn't monitor stdin as before.
Some test cases run test_mkfds directly. In such a case,
tests/functions.sh may connect stdin of test_mkfds to
/dev/null. /dev/null is always ready to be read.

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