Detect blank variable in trueorfalse

As a follow-on to I8cefb58f49dcd2cb2def8a5071d0892af520e7f7, put in
some detection around missing variable-to-test arguments in
trueorfalse.

Correct a couple of places where we were passing in blank strings,
resulting in the default always being applied.

Add test-cases and enhance the documentation a little.

Depends-On: I8cefb58f49dcd2cb2def8a5071d0892af520e7f7
Change-Id: Icc0eb3808a2b6583828d8c47f0af4181e7e2c75a
diff --git a/tools/fixup_stuff.sh b/tools/fixup_stuff.sh
index a601cf2..6ef32c8 100755
--- a/tools/fixup_stuff.sh
+++ b/tools/fixup_stuff.sh
@@ -108,7 +108,7 @@
         sudo setenforce 0
     fi
 
-    FORCE_FIREWALLD=$(trueorfalse False $FORCE_FIREWALLD)
+    FORCE_FIREWALLD=$(trueorfalse False FORCE_FIREWALLD)
     if [[ $FORCE_FIREWALLD == "False" ]]; then
         # On Fedora 20 firewalld interacts badly with libvirt and
         # slows things down significantly (this issue was fixed in