Makefile: Fail when doc recipes create dir named '$HOME'

This commit makes the `commanddocs` and `backenddocs` fail if they
accidentally create a directory named '$HOME'. This is basically a
regression test for issue rclone#8092.

It also makes those recipes rmdir the '$HOME/.config/rclone/'
directories. This will only delete empty directories, so nothing of
value should ever be deleted.

Author: dmcardle

Makefile

@@ -144,10 +144,14 @@ MANUAL.txt:	MANUAL.md
 	pandoc -s --from markdown-smart --to plain MANUAL.md -o MANUAL.txt
 
 commanddocs: rclone
+	-@rmdir -p '$$HOME/.config/rclone'
 	XDG_CACHE_HOME="" XDG_CONFIG_HOME="" HOME="\$$HOME" USER="\$$USER" rclone gendocs --config=/notfound docs/content/
+	@[ ! -e '$$HOME' ] || (echo 'Error: created unwanted directory named $$HOME' && exit 1)
 
 backenddocs: rclone bin/make_backend_docs.py
+	-@rmdir -p '$$HOME/.config/rclone'
 	XDG_CACHE_HOME="" XDG_CONFIG_HOME="" HOME="\$$HOME" USER="\$$USER" ./bin/make_backend_docs.py
+	@[ ! -e '$$HOME' ] || (echo 'Error: created unwanted directory named $$HOME' && exit 1)
 
 rcdocs: rclone
 	bin/make_rc_docs.sh