squid: add sysv initscript

Add SysV-style initscript, complete rewrite from
http://patchwork.ozlabs.org/patch/412057/



'stop' is handled by squid itself to gracefully (as possible) close
every pending connection and commit changes to disk. By default this is
configured for 30 seconds and can be configured via shutdown_lifetime in
/etc/squid.conf if someone is too anxious.
The script won't block until squid is properly shutdown - but people
should _REALLY_ use restart or reload if that's what they want, instead
of stop+start.

'restart' is handled by squid itself, since if we do a stop/start cycle
we must wait for a clean shutdown cycle (takes time).

'reload' is also handled by squid itself and it's not the same as
restart, it will just trigger a configuration reload without purging
runtime cache (RAM) contents.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>