doc: Update list of headers to use #include <...>.

[Collin Funk]

Now that sys/un.h might be replaced by Gnulib we have to keep in mind
the #include <...> vs. #include "..." issue.

I also noticed some other files in lib/*.in.h were missing from the
lists so I added them too.

First patch is adding them to the docs and second is adding them to
maint.mk. Separate patches since I'm not sure if docs or maint.mk are
back ported to stable branches. So it seemed less problematic that way.

Collin

0001-doc-Update-list-of-headers-to-use-include.patch
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0002-maintainer-makefile-Update-list-of-headers-to-use-in.patch
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