bootspec: honour profile number when sorting properly

bootspec: honour profile number when sorting properly

This corrects sorting of menu entries regarding profile numbers:

1. If the profile number is unset, let's treat this identical to profile
   0, when ordering stuff, because an item with no profile is
   conceptually the same as an item with only a profile 0.

2. Let's take the profile number into account also if sort keys are
   used. This was makes profiles work sensibly in type 1 entries, via
   the recently added "profile" stanza.

Follow-up for: 5fb90fa3194d998a971b21e4a643670ae5903f85