Ban <stddef.h> in kernel

<stddef.h> is replaced with <kernel/types.h>, which defines (s)size_t,
pid_t, and NULL as well as everything in <stdint.h> and <stdbool.h>
(these should be OK in kernel, probably).
Remove unnecessary includes
Don't include <stddef.h> from sigdef.h in kernel

If __KERNEL__ is defined (kernel/common.h), include <kernel/types.h>
instead of <stddef.h> for size_t.