OpenZFS restructuring - libspl

Factor Linux specific pieces out of libspl.

Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Reviewed-by: Sean Eric Fagan <sef@ixsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #9336

tests/zfs-tests/cmd/mkbusy/mkbusy.c

@@ -30,8 +30,6 @@
 #include <errno.h>
 #include <string.h>
 
-typedef enum boolean { B_FALSE, B_TRUE } boolean_t;
-
 static void
 usage(char *progname)
 {