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

[imp]

Summary

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

[imp]

regen after sys/_endian.h update

[imp]

rebase / update

[emaste]

exp-run?

[emaste]

(PR258544 endian.h exp-run)

[imp]

In D32051#724157, @emaste wrote:

(PR258544 endian.h exp-run)

Yes, this is paired with the endian.h changes because you need both to not @#^#@& some ports.

[markj]

include/byteswap.h

18

19

21

Used? Or present under /usr/include?

[jhb]

include/byteswap.h

19

(I included @markj's helpful comma as well)

21

I suspect used. I would perhaps say "included" instead of used?

[imp]

Update comments from review suggestiosn, and hopefully some additional verbosity
to disambiguate other possible readings not flagged in earlier reviews.

[markj]

include/byteswap.h

20

Not 100% sure this is what you meant.

[imp]

include/byteswap.h

20

Yes. <sys/_endian.h> provides all the traditional BSD/FreeBSD <sys/endian.h> interfaces, except the ones that interfere or are incompatible with glibc <endian.h>. Those bits are in either <endian.h>/<byteswap.h> or <sys/endian.h>. I struggled with how to convey that 'top level description' and would love any feedback you have. I like your change though.