byteswap.h: Add a glibc/linux compatible byteswap.h

For endian.h to work instead of sys/endian.h, some software needs
byteswap.h available. It must define {,}byteswap_{16,32,64}.
Included sys/_endian.h to get an appropriate byteswap16, etc
and defines the new macros in terms of them. Enhance _endian.h
to allow it to be included from here too.

Sponsored by: Netflix
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D32051