Networking: Fix accept() so that it returns the correct IP address for the selected socket IP domain.

net/socket and net/tcp: When we are ready to receive data or when we have outgoing data ready to be sent, we need to the notify the appropriate device driver of this condition.  The notifying socket will be configured as either PF_INET or PF_INET6 and must look up the correct driver using the correct IP domain.

include/net/if.h:  Add a bit to the device flags to indicate if the device packet buffer holds an IPv4 or an IPv6 domain packet.  Set/clear the flag along with the correct offset to the application payload data as each packet is received.
net/socket, net/tcp, net/udp:  Add logic to select the domain of the outputgoing packet before sending any UDP or TCP packet.  This sets the bit to indicate the IPv4 or IPv6 domain and the correct offset to the output going payload data.

Networking:  Remove field d_sndata from the device structure.  It is the same as d_appdata and unnecessary

Networking: UDP and TCP MSS depends on the IP header size (as well as the link layer header size) and cannot be represented with a single value.

Networking:  Replace all references to the macros net_ipaddr_copy, net_ipaddr_hdrcopy, net_ipaddr_cmp, net_ipaddr_hdrcmp, and net_ipaddr_maskcmp with the appropriate IPv4 or IPv6 version of the macro (such as net_ipv4addr_copy).  The goal is to support both IPv4 and IPv6 simultaneously.  This requires that the macros be distinct and not conditionally defined to one on or the other.

Networking:  Replace all references to net_ipaddr_t with either in_addr_t on net_ipv6addr_t.  The goal is to support both IPv4 and IPv6 simultaneously.  This requires that the two types be distinct and not conditionally typedef'ed to net_ipaddr_t.

Networking: Drivers can have both IPv4 and IPv6 addesses, but a socket can only only one or the other; The socket connnection structures need to include a union of IPv4 and IPv6 addresses for the local address binding and for the remote address connections

Networking:  More detangling of IPv6 logic.  Next steps will be more invasive and will get moved to a branch

Networking: Seperate tcp_input() and udp_input() into seprate functions tcp_ipv4_input(), tcp_ipv6_input(), udp_ipv4_input(), and upd_ipv6_input() than can deal will the data offsets caused by the differing sizes of the IP header.

Networking:  All Ethernet drivers:  Call ipv6_input if IPv6 is enabled and an IPv6 packet is received

Networking:  All Ethernet drivers:  Call ipv6_input if IPv6 is enabled and an IPv6 packet is received

- Copy net/devif/devif_input.c to ipv6_input.c.  Remove all IPv4-specific logic.
- Rename net/devif/devif_input.c to ipv4_input.c.  Remove all IPv6-specific logic
- Split IPv4 header structure out as net_ipv4hdr_s from net_iphdr_s

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.