emacspeak request: please update interactive spec for frameset-to-register

[T. V. Raman]

Could we update the interactive calling spec for
frameset-to-register
from
(interactive "cFrameset to register: \nP")

to
(interactive (list (register-read-with-preview
                      "Frame configuration to register: ")
                     current-prefix-arg))

This would save me having to explicitly advice the calling spec
for that command in Emacspeak.  It would also align the code with 
frame-configuration-to-register