snmpd init script uses start-stop-daemon without pidfile argument

The snmpd.init file in Precise, Quantal, Raring and Saucy invokes start-stop-daemon without the --pidfile argument. This causes all instances of snmpd to die when used on an LXC parent host, for example.

From the man page for start-stop-daemon:
       Note: unless --pidfile is specified, start-stop-daemon behaves similar
       to killall(1). start-stop-daemon will scan the process table looking
       for any processes which match the process name, uid, and/or gid (if
       specified). Any matching process will prevent --start from starting the
       daemon. All matching processes will be sent the TERM signal (or the one
       specified via --signal or --retry) if --stop is specified. For daemons
       which have long-lived children which need to live through a --stop, you
       must specify a pidfile.

In an LXC environment, all the container processes appear in the parent host's process table, thus "service snmpd {restart|stop}" will kill all running snmpd instances, even those in child containers.

In Precise, Quantal and Raring (5.4.3~dfsg-...), this is typical of how the process is stopped:
   start-stop-daemon --quiet --stop --oknodo --exec /usr/sbin/snmpd

In saucy (5.7.2~dfsg-4ubuntu1) this is typical of how the process is stopped:
   start-stop-daemon --quiet --stop --oknodo --exec /usr/sbin/snmpd && [ ! -f $SNMP_PID ] || rm $SNMP_PID

In all cases, --pidfile should be used to constrain the action of start-stop-daemon.

Observed empirically in snmpd-5.4.3~dfsg-2.4ubuntu1.1, observed in source packages for 5.4.3~dfsg-2.7ubuntu and 5.7.2~dfsg-4ubuntu1 on launchpad.