Move frameset-to-register stuff from register.el to frameset.el.

Move frameset-to-register stuff from register.el to frameset.el.

lisp/register.el (frameset-frame-id, frameset-frame-with-id, frameset-p)
(frameset-restore, frameset-save, frameset-session-filter-alist):
Remove declarations.
(register-alist): Doc fix.
(frameset-to-register): Move to frameset.el.
(jump-to-register, describe-register-1): Remove frameset-specific code.

lisp/frameset.el (frameset-p): Add autoload cookie.
(frameset--jump-to-register): New function, based on code moved from
register.el.
(frameset-to-register): Move from register.el.  Adapt to `registerv'.