invent menu fixes (mostly menustyle:Traditional)

invent menu fixes (mostly menustyle:Traditional)

For menustyle:Traditional, the object class prompt for 'D' includes
an entry choice of 'm' to request a menu.  Supplying real classes
and 'm' resulted in a menu limited to those classes, as intended,
but any of BUCX for curse/bless state and 'm' without any actual
classes resulted in a menu of entire invent.  A one-line fix once
the proper place for that fix was located.

For menustyle:Traditional or menustyle:Combination, 'A' includes an
extra choice of 'i' to examine relevant inventory prior to choosing
object classes (and object identification also offers that), but
the inventory display showed everything rather than just the items
applicable to 'A' (or to object ID).  Figuring out where to apply
the fix was trivial, but the fix itself was a bit more involved and
it exposed a latent bug in display_pickinv(): "" was supposed to be
the same as NULL when passed in as list of target inventory letters,
but it wasn't being handled correctly.