Documentation/watchdog: Fix the file descriptor leak when no cmdline arg given

we start a infinite loop when user gives ./watchdog-test, and when user
ctrl + c's the program, we just exit immeadiately with out closing the
filedescriptor of the watchdog device. a signal handler is used to
do the job of closing the filedescriptor and exiting the program.

Signed-off-by: Devendra Naga <devendra.aaru@gmail.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>