mod.h: make consistent across all three platforms

mod.h only exists to include the platform-specific mod_os.h, so we can
get rid of it and just call the platform header mod.h.

Then, create a libspl mod.h, and move the relevant items to it so we can
start building on it.

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 #17537