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>