* Simplify EINTR/ECANCEL error handling
1. Add semaphore uninterruptible wait function
2 .Replace semaphore wait loop with a single uninterruptible wait
3. Replace all sem_xxx to nxsem_xxx
* Unify the void cast usage
1. Remove void cast for function because many place ignore the returned value witout cast
2. Replace void cast for variable with UNUSED macro
Squashed commit of the following:
net/icmpv6: IPPROT_ICMP6 socket logic now builds without error.
net/icmpv6: Add support for read-ahead and poll(). Initial commit is just cloned from ICMP with the appropriate name changes.
configs/: All defconfig filess that include CONFIG_NET_ICMPv6_SOCKET=y need to select CONFIG_SYSTEM_PING6=y and deselect CONFIG_DISABLE_POLL.
Update NSH documention to show that ping6 is now a built in command.
net/icmpv6: Add icmpv6_sendto.c and icmpv6_recvfrom.c. Initial versions are just clones from icmp/ with appropriate name changes.
net/icmpv6: Clone some ICMP socket logic as the beginning of support for ICMPv6 socket support.
Rename CONFIG_NET_ICMPv6_PING to CONFIG_NET_ICMPv6_SOCKET. Move prototype for icmpv6_ping from include/nuttx/net/icmpv6 to net/icmpv6/icmpv6.h
