gdb: change set_internalvar_function to take a unique pointer

This makes the transfer of ownership a bit clearer, even though the
internal_function is still held with a raw pointer inside internalval.

Change-Id: Ie8d13270b64737b92291532acfbfcbc992b482b5
Reviewed-By: Guinevere Larsen <guinevere@redhat.com>