init.rc: allow IPPROTO_ICMP support

Allow userspace programs to create IPPROTO_ICMP sockets.

This socket type allows an unprivileged program to safely
send ICMP_ECHO messages and receive the corresponding
ICMP_ECHOREPLY messages, without relying on raw sockets or
setuid programs.

Please see http://lwn.net/Articles/443051/ for details.

In particular, this allows us to use a version of ping
which doesn't have any capabilities
(https://android-review.googlesource.com/52072).
In addition, this allows us to safely implement an IPv4 ICMP
based version of InetAddress.isReachable()
(https://code.google.com/p/android/issues/detail?id=20106)

Change-Id: I876718151efa8219c4f34f573e35e21256fe2316