Improve Guide chapter 7

 * Typos/grammar
 * Fix breakdown of contract error message
 * Show `struct/dc` instead of deprecated lazy contracts
Fix broken link
Another small contract fix
Add DNS tests
Minor fix in Guide chapter 11
slideshow/balloon: add `balloon-enable-3d'
doc repair
Fix `configure' for OpenBSD

"-rdynamic" doesn't work with GCC 4.7 on OpenBSD: "cc: error:
unrecognized command line option '-rdynamic'". The switch isn't
necessary because it is only a wrapper to "-Wl,--export-dynamic".
Look GCC bug 37454.
add 'so-mode to `system-type'

To work better with OpenBSD (which has been a problem since we
changed `ffi-lib' to open libraries in "local" mode by default).
openssl: remove special handling of OpenBSD

This special handling is now built into `ffi-lib'.
doc correction and test additions for `atan'

Closes PR 10324
tweak docs
Changed `define-union-language' to merge nonterminals and their
right-hand-sides instead of causing an error when more than one language
in the union defines the same nonterminal.
follow up to William J. Bowman's commit

- put the tests together with the other define-union-language tests
- fix the docs
- add a release note
- construct the merged language a bit more directly
- properly deal with this kind of thing:
  (define-language L1 (e f ::= 1 2 3))
  (define-language L2 (e g ::= 4 5 6))
  (define-union-language L L1 L2)
used the wrong dir when checking the info.rkt santity

thanks to Jos Koot for pointing out this bug
Start to add internal documentation
Support querying AAAA records for IPv6
Shorten IPv6 addresses for display
Support reverse lookups with IPv6 addresses
Set the socket protocol family

Allows the use of an IPv6 nameserver