Move check_file to os/$platform section

Keep check_file_generic() in shared code base, and allow special case
code in check_file() in os section. In future, macOS will have
additional checks in check_file().

Linux and FreeBSD wrappers just calls check_file_generic().

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Jorgen Lundman <lundman@lundman.net>
Closes #12385