Do not allow a command in an account-hook to trigger another account-hook.

Do not allow a command in an account-hook to trigger another account-hook.
Recent changes in the IMAP path canonifier mean that account-hooks
that set variables to URLs (eg folder, spoolfile) can trigger a
recursive account-hook. Now we just bail out of account-hook early if
we are called recursively, but perhaps we should warn the user to use
a folder-hook instead.
Thanks to Kyle Wheeler for finding this one.