This change adds support for semi-standard IPPROTO_ICMP AF_INET datagram sockets. This replaces the old ad hoc, nonstandard way of implementing ping with a more standard, socket interface.

Squashed commit of the following:

    net/icmp:  Finishes off icmp_recvfrom().
    net/icmp:  Add readahead support for IPPROTO_ICMP sockets.
    net/icmp:  Add poll() support for IPPROTO_ICMP sockets.
    net/icmp:  Add a connection structure for IPPROTO_ICMP sockets.
    net/icmp:  Implements sendto for the IPPROTO_ICMP socket.
    net/icmp:  Move icmp_sendto() and icmp_recvfrom() to separate files.  They are likely to be complex (when they are implemented).
    net/icmp:  Hook IPPROTO_ICMP sockets into network.  Fix some naming collisions.  Still missing basic ICMP send/receive logic.
    configs: apps/system/ping current need poll() enabled.
    configs: All defconfig files that use to enable low-level support must now enabled CONFIG_SYSTEM_PING.
    net/icmp:  Adds basic build framework to support IPPROTO_ICMP sockets.

net/socket/socket.c

@@ -60,7 +60,7 @@
  *   socket() creates an endpoint for communication and returns a socket
  *   structure.
  *
- * Parameters:
+ * Input Parameters:
  *   domain   (see sys/socket.h)
  *   type     (see sys/socket.h)
  *   protocol (see sys/socket.h)
@@ -134,7 +134,7 @@ int psock_socket(int domain, int type, int protocol, FAR struct socket *psock)
 
   /* Get the socket interface */
 
-  sockif = net_sockif(domain);
+  sockif = net_sockif(domain, type, protocol);
   if (sockif == NULL)
     {
       nerr("ERROR: socket address family unsupported: %d\n", domain);