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

include/sys/zfs_context.h

@@ -204,18 +204,6 @@ extern void vpanic(const char *, va_list)
 #endif	/* DTRACE_PROBE4 */
 #define	DTRACE_PROBE4(a, b, c, d, e, f, g, h, i)
 
-/*
- * Tunables.
- */
-typedef struct zfs_kernel_param {
-	const char *name;	/* unused stub */
-} zfs_kernel_param_t;
-
-#define	ZFS_MODULE_PARAM(scope_prefix, name_prefix, name, type, perm, desc)
-#define	ZFS_MODULE_PARAM_ARGS void
-#define	ZFS_MODULE_PARAM_CALL(scope_prefix, name_prefix, name, setfunc, \
-	getfunc, perm, desc)
-
 /*
  * Threads.
  */