libspl: implement ZFS_MODULE_PARAM for userspace

For each tunable declaration, we create a zfs_tunable_t with its
details, and then a pointer to it in the 'zfs_tunables' ELF section,
that we can access later with a little support from the linker.

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

lib/libspl/include/Makefile.am

@@ -59,6 +59,7 @@ libspl_sys_HEADERS = \
 	%D%/sys/time.h \
 	%D%/sys/trace_spl.h \
 	%D%/sys/trace_zfs.h \
+	%D%/sys/tunables.h \
 	%D%/sys/types.h \
 	%D%/sys/types32.h \
 	%D%/sys/uio.h \