Move timeval struct into its own header and include it from headers where

Move timeval struct into its own header and include it from headers where
needed or defined by standards plus from sys/types.h. This makes several
pkgsrc packages more happy.

Including just sys/types.h to use struct timeval works there in every BSD,
Linux and Solaris, so we can't just ignore it.

Also make struct timeval standards compilant in progress.

Inspired-by: FreeBSD