lzc: move lzc_ioctl_fd() into lzc proper

Name the OS-specific call lzc_ioctl_fd_os(), and make lzc_ioctl_fd()
wrap it, so we can do more in the wrapper.

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #17344