Better handling if parent closes REDO_CHEATFDS or MAKEFLAGS fds.

Silently recover if REDO_CHEATFDS file descriptors are closed, because
they aren't completely essential and MAKEFLAGS-related warnings already
get printed if all file descriptors have been closed.

If MAKEFLAGS --jobserver-auth flags are closed, improve the error
message so that a) it's a normal error instead of an exception and b)
we link to documentation about why it happens.  Also write some more
detailed documentation about what's going on here.

docs/FAQInterop.md

@@ -51,5 +51,3 @@ dependencies are too generous, we end up calling 'make all'
 more often than necessary.  But 'make all' probably runs
 pretty fast when there's nothing to do, so that's not so
 bad.
-
-

docs/FAQParallel.md

@@ -156,3 +156,56 @@ Yes.  Put this in your .do script:
 	
 The child makes will then not have access to the jobserver,
 so will build serially instead.
+
+
+<a name='MAKEFLAGS'></a>
+# What does the "broken --jobserver-auth" error mean?
+
+redo (and GNU make) use the `MAKEFLAGS` environment variable to pass
+information about the parallel build environment from one process to the
+next.  Inside `MAKEFLAGS` is a string that looks like either
+`--jobserver-auth=X,Y` or `--jobserver-fds=X,Y`, depending on the version of
+make.
+
+If redo finds one of these strings, but the file descriptors named by `X`
+and `Y` are *not* available in the subprocess, that means some ill-behaved
+parent process has closed them.  This prevents parallelism from working, so
+redo aborts to let you know something is seriously wrong.
+
+GNU make will intentionally close these file descriptors if you write a
+Makefile rule that contains *neither* the exact string `$(MAKE)` nor a
+leading `+` character.  So you might have had a Makefile rule that looked
+like this:
+
+	subdir/all:
+		$(MAKE) -C subdir all
+
+and that worked as expected: the sub-make inherited your parallelism
+settings.  But people are sometimes surprised to find that this doesn't work
+as expected:
+
+	subdir/all:
+		make -C subdir all
+
+In that case, the sub-make does *not* inherit the jobserver file
+descriptors, so it runs serially.  If for some reason you don't want to use
+`$(MAKE)` but you do want parallelism, you need to write something like this
+instead:
+
+	subdir/all:
+		+make -C subdir all
+
+And similarly, if you recurse into redo instead of make, you need the same
+trick:
+
+	subdir/all:
+		+redo subdir/all
+
+There are a few other programs that also close file descriptors.  For
+example, if your .do file starts with `#!/usr/bin/env xonsh`, you might
+run into [a bug in xonsh where it closes file descriptors
+incorrectly](https://github.com/xonsh/xonsh/issues/2984).
+
+If you really can't stop your program from closing file descriptors that it
+shouldn't, you can work around the problem by unsetting `MAKEFLAGS`.  This
+will let your program build, but will disable parallelism.

docs/extra_style.css

@@ -1,7 +1,7 @@
 p code {
     border: 0;
     padding: 0 2px 0 2px;
-    font-size: 100%;
+    font-size: 80%;
     white-space: nowrap;
     color: #a04040;
     letter-spacing: -1px;