app-antivirus/clamav-unofficial-sigs: new revision with a cron job.

app-antivirus/clamav-unofficial-sigs: new revision with a cron job.

This update script needs to run as a restricted user, with bash as its
shell, and with a real home directory to prevent cron errors in the
logs. The best way to do that seems to be to install a custom job
into /etc/cron.d that sets the HOME and SHELL variables.

This new revision provides a cron job that works as documentation, and
adds a new USE=cron flag to install it. The default is NOT to install
it because /etc/cron.d is not 100% standard, and the update script
needs to be configured before it will work.

Closes: https://bugs.gentoo.org/694054
Package-Manager: Portage-2.3.76, Repoman-2.3.16
Signed-off-by: Michael Orlitzky <mjo@gentoo.org>