Use `any/c` for the `ip-address-string?` domain

Previously, it was `string?` which is needlessly
restrictive and caused premature contract failures.

Please merge into 5.3.4
(cherry picked from commit 5735d37794315dd9f6d2dedfdf6cae3a1dc7181b)
fix improper fraction display

closes PR 13558

please include in the release
(cherry picked from commit d9adb4e6e987124571f043a28be682b465435aa7)
fix test suite to match earlier fix to obligation info

merge to release branch, please
(cherry picked from commit 1ebf66252f2e3638506c347f247956d01bac45c7)
Better error message for non-existent generics groups.

Closes PR13671.
(cherry picked from commit aac22e1ce5210be594d58f30d223de3715e2392a)
redex: bug fix for gerating terms including #f

please include in the release
(cherry picked from commit 495e5c94cf85b76c25c4bfe9e98f1ae916abd0b4)
fix a non-deterministic test
(cherry picked from commit bc5c89cbf076841f12a2a8ca9de112e8a1ac8aba)
adjust pick-a-maze based on Neil's comments

please include in the release
(cherry picked from commit f093131607b05905774ae32c54791dab2f02715b)
Tests for UDP multicast primitives
(cherry picked from commit 94e36c8193f8e0ec733a76004bb88a54f0d4921d)
fix `for/flvector', etc. unsafety

Closes PR 13675

Merge to v5.3.4
(cherry picked from commit 8b5523bd10d3d632374c1fd90fe1f513eac13353)
Change doc/.gitignore to make git aware of files with non-generated contents.
(cherry picked from commit 1bf9e124ebffae8e4e5426594d4c3334c9d4e51a)
doc clarification
(cherry picked from commit 8d7c231cd8441079396e7c9dd5894f807af73dce)
fix `raco setup -j' for a non-places build

When places are not enabled, then `raco setup' uses multiple
Racket processes that commuincate via pipes. Don't try to send
a place channel over a pipe.

Merge to v5.3.4
(cherry picked from commit 2295d16074b9538ce500cf9bf9de65265ce5531c)
Correct typo.
(cherry picked from commit 075212d07c1c753c9ddc72ead6aa97fa9193304e)
plai/{gc2/}mutator: doc fixes

A symptom of mismatched `for-label' imports was that searching for "<="
would show red-underlined links to the `plai/{gc2/}mutator' documentation.

Merge to v5.3.4
(cherry picked from commit de9b77ad6d68df749ba23f77b273d1b42b06bc86)
reference: simplify some contractcs
(cherry picked from commit 7c0f35e1388c408cda2f965ca48a337cfad4e83a)
racket/udp: fixed for `udp-bind!' and `udp-connect!'

Fix ephemeral-port support in `udp-bind!', and
change `udp-bind!' and `udp-connect!' to try address
resolutions in order to find one that works (which is
typically needed to auto-select an IPv4 or IPv6 variant
of an address).
(cherry picked from commit 842da32e4872860493d04b33d22f3385034319f3)
fix a UDP multicast test to make it work on Windows XP
(cherry picked from commit 6e0492dff0813b1c305398f9607bd31491645b46)
redex: elminate another #f/failure confusion
(cherry picked from commit 468d2b192e833747feaa3cac0726bac515c11ddb)
fix hole in validator

The validator was not as smart as the compiler in determining
that a `let' expression could be relied on to produce a
constant-shaped function (without side effect or error) in the case
that a right-hand side expression is a call to a known structure
constructor or predicate.

Closes PR 13679

Merge to v5.3.4
(cherry picked from commit e49956e3ea85d3e6a2a1d1e3be5edcb9115a537e)
Make configure install the docs in the standard place.

Closes #315 (on github)
(cherry picked from commit 91aa7f3527ee1012266688bf6ce471e5240d1921)