debugfs: move to new strtobool

No functional changes requires that we eat errors from strtobool.
If people want to not do this, then it should be fixed at a later date.

V2: Simplification suggested by Rusty Russell removes the need for
additional variable ret.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>