<stdint.h>: Move out ptrdiff_t type.

<stdint.h>: Move out ptrdiff_t type.

 The ISO C does not specify that <stdint.h> should provide this type,
 this type should be declared in <stddef.h> for use by userland.

 Since we do not want to create a separate header for this useful type
 to be used by kernel, move it to <sys/types.h> under _KERNEL visibility.

 Adjust few places to include <stddef.h> with a minor whitespace cleanup.