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.

t/205-readonly/.gitignore

@@ -0,0 +1 @@
+/rodir

t/205-readonly/all.do

@@ -0,0 +1,25 @@
+[ -e rodir ] && chmod u+w rodir
+[ -e rodir/rwdir ] && chmod u+w rodir/rwdir
+rm -rf rodir
+mkdir rodir rodir/rwdir
+
+cd rodir
+cat >default.ro1.do <<-EOF
+	chmod u+w "\$(dirname "\$1")"
+	echo 'redir' >\$3
+EOF
+cat >default.ro2.do <<-EOF
+	chmod u+w "\$(dirname "\$1")"
+	echo 'stdout'
+EOF
+
+# Check that:
+#  - redo works when the .do file is in a read-only directory.
+#  - redo works when the target is in a read-only directory that becomes
+#    writable only *after* launching the .do script. (For example, the .do
+#    might mount a new read-write filesystem in an otherwise read-only
+#    tree.)
+chmod a-w . rwdir
+redo rwdir/a.ro1
+chmod a-w . rwdir
+redo rwdir/a.ro2

t/205-readonly/clean.do

@@ -0,0 +1,4 @@
+[ -e rodir ] && chmod u+w rodir
+[ -e rodir/rwdir ] && chmod u+w rodir/rwdir
+rm -rf rodir
+rm -f *~ .*~