Allow the use of global config file

If not found elsewhere, look for the config file in /etc/todo/config.
`make install` installs a global config file, setting TODO_DIR=~/.todo.
todo.sh creates TODO_DIR if it doesn't exist.

tests/t2120-shorthelp.sh

@@ -45,6 +45,9 @@ mv todo.cfg custom.cfg
 mv .todo.actions.d custom.actions
 echo 'export TODO_ACTIONS_DIR=$HOME/custom.actions' >> custom.cfg
 
+# Avoid the use of global config file, if it exists
+export TODOTXT_GLOBAL_CFG_FILE=global.cfg
+
 test_todo_session '-h and fatal error without config' <<EOF
 >>> todo.sh -h | sed '/^ \\{0,2\\}[A-Z]/!d'
   Usage: todo.sh [-fhpantvV] [-d todo_config] action [task_number] [task_description]