Add ushort and ulong to linux/types.h.

When porting code once written for Linux we find not only uints but also ushort and ulong.
Provide central typedefs as part of the linuxkpi for those as well.

Reviewed by: hselasky, emaste
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D19405