Networking: (1) Copied all ICMP sources files to net/icmpv6 with proper...

Networking:  (1) Copied all ICMP sources files to net/icmpv6 with proper renaming and removal of IPv4 logic, (2) remove IPv6 logic from files in net/icmp, (3) copied include/nuttx/icmp.h to icmpv6.h and removed IPv4 specific logic, (4) removed all IPv6 logic from icmp.h, (5) IP_HDRLEN became IPv4_HDRLEN and IPv6_HDRLEN, (6) ip_chksum() became ipv4_chksum() and ipv6_chksum(), and (7) added partial support for ICMPv6 statistics.