sysctl: make sys/sysctl.h self contained

sys/sysctl.h only needs u_int and size_t from sys/types.h. When the
sysctl interface was designed, having one more more prerequisites
(especially sys/types.h) was the norm. Times have changed, and to make
things more portable, make sys/types.h optional. We do this by including
sys/_types.h, defining size_t if needed, and changing u_int to 'unsigned
int' in a prototype for userland builds. For kernel builds, sys/types.h
is still required.

Sponsored by:		Netflix
Reviewed by:		kib, jhb
Differential Revision:	https://reviews.freebsd.org/D31827

sys/sys/sysctl.h

@@ -1180,9 +1180,14 @@ struct sbuf *sbuf_new_for_sysctl(struct sbuf *, char *, int,
 	    struct sysctl_req *);
 #else	/* !_KERNEL */
 #include <sys/cdefs.h>
+#include <sys/_types.h>
+#ifndef _SIZE_T_DECLARED
+typedef	__size_t	size_t;
+#define	_SIZE_T_DECLARED
+#endif
 
 __BEGIN_DECLS
-int	sysctl(const int *, u_int, void *, size_t *, const void *, size_t);
+int	sysctl(const int *, unsigned int, void *, size_t *, const void *, size_t);
 int	sysctlbyname(const char *, void *, size_t *, const void *, size_t);
 int	sysctlnametomib(const char *, int *, size_t *);
 __END_DECLS