racket/sandbox: add `sandbox-run-submodules`
openssl: allow `ssl-abandon-port` on input ports

Make `ssl-abandon-port` consistent with the docs and with
`tcp-abandon-port`.
net/url: fix problems with new implementation on `net/http-client`

Various `net/url` operations need to request a close of the
connection after the operation completes, and `het/http-client`
needs to actually close the input-port half of a connection.

Also, add `http-conn-abandon!`.

Also also, add limits on internal pipes, so that data doesn't pile
up in a connection-processing thread, and fix POST/PUT by adding
a needed CRLF after posted data.
Fix link in unstable docs.
Improve type for generic-set?.
fix for abc7f9a873
fix string-contains?

closes PR 13990
Fix type of generic-set? for hash set default.
fix multi-file drag-and-drop on Unix

Thanks to Laurent Orseau.
scribble/base: fix problem with macro-introduced `include-section'
include license with each package
licenses for native packages
Direct people to temporary snapshot pages.
add a case-> contract example
Removing "collects".

I don't see how it's useful to have it, and I keep getting drdr
notifications about it being bogus.
Adjust the DMdA teachpacks to be found via info.rkt files.

This follows commit efd26833015a41706b1e26a8b13efda9ada0eb2b for the
HtDP languages.
adjust some error messages based on new conventions
add custom writers for a bunch of contracts
Remove unnecessary #:proc forms in TR unit tests.
Remove reset-unboxed-bindings and unboxed-gensym.