apps/netutils/dnsclient:  Can select to be either IPv4 or IPv6, but not both (IPv6 still does not compile)

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.

This is not the official configuration for the PX4 board and has led to confusion by NuttX users.  The board configuration also requires some ongoing maintenance and customization to support ongoing PX4 testing and evaluation.  It is best retained the PX4 repositories where it can be properly maintained and not in the upstream NuttX repository.

Signed-off-by: Marco Krahl <ocram.lhark@gmail.com>

apps/system/composite:  Fix some corrupted conditional compilation that crept in.  Looks like some automatated update gone awry.

apps/system/usbmsc:  Fix some corrupted conditional compilation that crept in.  Looks like some automatated update gone awry.

STM32 LTDC: Move ltdc.h from include/nuttx/video to arch/arm/include/stm32; Trivial updates after general review