MI, not required to be a fixed size, and used in multiple headers.
This will grow in time, as more things move here from <sys/types.h>
and <machine/ansi.h>.
o Add missing type definitions (uint16_t and uint32_t) to
<arpa/inet.h> and <netinet/in.h>.
o Reduce pollution in <sys/types.h> by using `#if _FOO_T_DECLARED'
widgets to avoid including <sys/stdint.h>.
o Add some missing type definitions to <unistd.h> and note the ones
that still need to be added.
o Make use of <sys/_types.h> primitives in <grp.h> and <sys/types.h>.
Reviewed by: bde
beecc37c73