Task #4643: use THROW_SYSCALL() macro instead of 'throw SystemCallException(...)' to ensure that errno is loaded immediately and not after it was possible clobbered by string() or Backtrace(). Replace open-coded version in a few instances where the problem had already bitten. Note that all pthread_*() do not set errno, but return it (or 0), so keep those as-is. Fix up errno passing of pthread_kill() in this regard.