common: forward_handler() for handlers with move-only args

common: forward_handler() for handlers with move-only args

executors will always call the handler's lvalue overload of operator(),
so forward_handler() is required to wrap calls to bind_handler() with
move-only argument types. Completion uses this to forward all of its
CompletionHandlers

Signed-off-by: Casey Bodley <cbodley@redhat.com>