examples: various fixes in ephiperfifo.c

The main change here is the timer value that was wrong, it was given in
usecs (ms * 1000), while the itimerspec struct wants nsecs (ms * 1000 *
1000). This resulted in the callback being invoked WAY TOO OFTEN.

As a quick check you can run this command before and after applying this
commit:

    # shell 1
    ./ephiperfifo 2>&1 | tee ephiperfifo.log
    # shell 2
    echo http://hacking.elboulangero.com

 > hiper.fifo

Then just compare the size of the logs files.

Closes #3633
Fixes #3632
Signed-off-by: Arnaud Rebillout <arnaud.rebillout@collabora.com>