perf script: Add drop monitor script

A while back I created the dropmonitor protocol, which allowed users to get
reports of dropped frames communicated to them via a netlink socket.

While useful, several people have now asked that I integrate the ability
to do drop monitoring with perf, so they don't have to run additional
tools.

This patch adds a drop monitor script to the perf suite, and provides
the same output that the netlink socket does.

Cc: Ingo Molnar <mingo@elte.hu>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1309801217-22450-1-git-send-email-nhorman@tuxdriver.com
Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>