Optional PCP crontab installation

For Linux systems where systemd(1) is active, the polling and daily
scripts needed to manage service health, PCP archives, log rotation and
related matters are handled by .timer "services" that are controlled
by systemctl(1) and no manual cron(1) configuration is needed.

For non-Linux systems, this work is done by cron(1) for the user
"pcp", using crontab entries installed in the right place by the
PCP package installation procedures and again no manual cron(1)
configuration is needed.

For Linux systems where systemd(1) is not active, some manual
configuration is required to allow cron(1) to do the work.
Specifically something along the lines of:

    $ cd /usr/share/pcp/cron.d
    $ sudo crontab -u pcp -r
    $ cat pcp-* | sudo crontab -u pcp -

and then check with:

    $ sudo crontab -u pcp -l

and customize (e.g. comment out or remove lines for services you're
not using) with:

    $ sudo crontab -u pcp -e
