boolean_t: change to unsigned int to avoid signed bitfield warnings

This is the final part, which actually makes boolean_t unsigned. Note
that we do not change its size, nor do we try to change it directly to
bool, since that results in a lot of regressions.

Converting the remaining instances of boolean_t to plain C99 bool can
now be done in a piecemeal fashion, after which boolean_t may hopefully
be retired.

MFC after:	1 week
Reviewed by:	jhb
Differential Revision: https://reviews.freebsd.org/D39753

sys/sys/types.h

@@ -273,7 +273,7 @@ typedef __rman_res_t    rman_res_t;
 typedef __register_t	syscallarg_t;
 
 #ifdef _KERNEL
-typedef	int		boolean_t;
+typedef	unsigned int	boolean_t;
 typedef	struct _device	*device_t;
 typedef	__intfptr_t	intfptr_t;