1. For buffered tcp/udp case, if CONFIG_NET_ARP_SEND/CONFIG_NET_ARP_IPIN/CONFIG_NET_ICMPv6_NEIGHBOR isn't enabled and the table doesn't contain ip<->ethaddr mapping yet, the logic will skip the realtransmission and then arp/neighbor can't steal the final buffer to generate arp/icmpv6 packet.
2.for all other case, the tcp layer or user program should already contain the retransmit logic, the check is redundancy and may generate many duplicated packets if arp/icmpv6 response is too slow because the cursor stop forward. If user still concern about the very first packet lost, he could fix the issue by enabling CONFIG_NET_ARP_SEND/CONFIG_NET_ICMPv6_NEIGHBOR at begin.
Squashed commit of the following:
sched: Rename all use of system_t to clock_t.
syscall: Rename all use of system_t to clock_t.
net: Rename all use of system_t to clock_t.
libs: Rename all use of system_t to clock_t.
fs: Rename all use of system_t to clock_t.
drivers: Rename all use of system_t to clock_t.
arch: Rename all use of system_t to clock_t.
include: Remove definition of systime_t; rename all use of system_t to clock_t.
sched/semaphore: Add nxsem_post() which is identical to sem_post() except that it never modifies the errno variable. Changed all references to sem_post in the OS to nxsem_post().
sched/semaphore: Add nxsem_destroy() which is identical to sem_destroy() except that it never modifies the errno variable. Changed all references to sem_destroy() in the OS to nxsem_destroy().
libc/semaphore and sched/semaphore: Add nxsem_getprotocol() and nxsem_setprotocola which are identical to sem_getprotocol() and set_setprotocol() except that they never modifies the errno variable. Changed all references to sem_setprotocol in the OS to nxsem_setprotocol(). sem_getprotocol() was not used in the OS
libc/semaphore: Add nxsem_getvalue() which is identical to sem_getvalue() except that it never modifies the errno variable. Changed all references to sem_getvalue in the OS to nxsem_getvalue().
sched/semaphore: Rename all internal private functions from sem_xyz to nxsem_xyz. The sem_ prefix is (will be) reserved only for the application semaphore interfaces.
libc/semaphore: Add nxsem_init() which is identical to sem_init() except that it never modifies the errno variable. Changed all references to sem_init in the OS to nxsem_init().
sched/semaphore: Rename sem_tickwait() to nxsem_tickwait() so that it is clear this is an internal OS function.
sched/semaphoate: Rename sem_reset() to nxsem_reset() so that it is clear this is an internal OS function.
net/socket/recvfrom.c: Check fromlen integrity before using it.
net/socket/net_sockets.c: Always check for valid psock before using.
net/tcp/tcp_send_unbuffered.c: Avoid using psock beforing checking its integrity.
sched/timer/timer_create.c: Fix watchdog resource leak if cannot allocate a new timer.
