Add linux kernel module support

Setup linux kernel module support, this includes:
- zfs context for kernel/user
- kernel module build system integration
- kernel module macros
- kernel module symbol export
- kernel module options

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

module/zfs/include/sys/spa_impl.h

@@ -226,7 +226,7 @@ struct spa {
 	refcount_t	spa_refcount;		/* number of opens */
 };
 
-extern const char *spa_config_path;
+extern char *spa_config_path;
 
 #ifdef	__cplusplus
 }