squid: Rework initscript

squid: Rework initscript

The initscript now takes care that the squid proxy server process
is properly shut down. If that fails, it will remove the cache
index and let it be recreated at the next start. A warning is
shown to the user.

The "flush" command will now remove the entire proxy cache.

Signed-off-by: Matthias Fischer <matthias.fischer@ipfire.org>
Signed-off-by: Arne Fitzenreiter <arne.fitzenreiter@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>