Mac OS X: make --disable-libffi the default

Avoids a common problem with libffi installed by MacPorts
causing problems with a mismatch between an iconv installed
by MacPorts and the system iconv. (When libffi is installed,
then -I/opt/include for the libffi heads also picks up the
iconv headers, but the ordering of the lib flags doesn't
pick up libiconv from /opt/lib. We could try to hack around
this by ordering the flags just right, but it seems better
to avoid the issue.)
raco exe: handle failing submodule search correctly

An attempt to detect a submodule could trigger the original module
name resolver when the would-be enclosing module would be handled
by the embedding-specific resolver. When a submodule is not found
but its would-be enclosing module is embedded, then assume that
the default resolver wouldn't find the submodule, eithe --- and
therefore avoid a potential "collection not found" error.
Scribble overview: mention tables comments, and pictures
syntax/parse/experimental/template: quasitemplate, template/loc
Adjust the web download phase of the install .plt file
menu item in drracket so that it first checks to see if
the downloaded file matches a .plt archive and, if not,
try to put some friendlier message in front of the gzip error

closes PR 9425
closes PR 13129
Reference: fix interface example typo
Synch German string constants with latest.
Squelching port-closed error messages
fix redex pdf generation when a string is passed
Moved default location for compiled-root for easier tab-completion of "collects/".
when PLTPICTBOX is set, include "Insert Pict Box" in DrRacket

Works well enoough for demos, at least.
fix doc bug for `gauge%'
ffi/unsafe doc repairs

Closes PR 13143
fixes suggested by latest reviewer of the hosc submission
fix module browser's red coloring of boxes in the
case that a file is required as x.ss, but its name
is actually x.rkt (and similar situations)

related to PR 13080

Also, Rackety
clean up indentation and move arrows to the front, while I'm here
fix rectangle computation for shortcircuiting drawing
in the module browser

closes PR 13080
syntax/parse/experimental/template: make errors consistent with 'syntax'
added unstable/custom-write
Fix type->contract on structures. Now uses struct/c.