Disallow prefix modifier on variables with composite values

[keanu-delgado]

One last proposal. This one is pretty explicit in the spec.

From https://tools.ietf.org/html/rfc6570#section-2.4.1 :

   A prefix modifier indicates that the variable expansion is limited to
   a prefix of the variable's value string.  Prefix modifiers are often
   used to partition an identifier space hierarchically, as is common in
   reference indices and hash-based storage.  It also serves to limit
   the expanded value to a maximum number of characters.  Prefix
   modifiers are not applicable to variables that have composite values.

[rodneyrehm]

looks good, thank you!