docs: mention cygwin shortcoming in <sys/un.h>

I'm not quite ready to implement a replacement <sys/un.h> that
automatically works around this problem, since making the replacement
work for mingw seems much harder; so for now I'm just documenting
the issue I hit.

* doc/posix-headers/sys_un.texi (sys/un.h): Mention problem.

Signed-off-by: Eric Blake <eblake@redhat.com>