raco pkg: fix access of 'default version info

Includes repair to `raco pkg catalog-show` to use the 'version table
in a package description.
OpenBSD: another approach to finding stack bounds

OpenBSD provides pthread_stackseg_np(), which directly reports
the stack-bounds information that Racket needs, so we can use
that instead of the approach used on other Unix variants. The
approach used for other Unix variants seems not to work for OpenBSD
because the stack location at the point that main() is called
is already significantly far from the stack base (on the order
of 100k on a 32-bit system in my test using OpenBSD 5.2).
adjust scribble html renderer to fall back from svg to png
and vice-versa
add svg-bytes to the file/convertible docs as a standard request

also, bring convertible.scrbl down below 102 columns
support svg-bytes for 2htdp/image images
adjust image docs to use svg instead of png
Base now works, but was broken before
Racket now works, but was broken before
Re-enabling DrDr test of base
Enabling tests and fixing them
Fixing PR14096
Moved this to the wrong place
Fixing PR14098
Restoring old decoding behavior of net/url and allowing future compatibility by parameterizing what is accepted
Adding helpful print out
Fixing PR14062
Adding noun re PR13885
Another friendly error message for weak sauce users
Fixing PR14088 by showing entire path
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