Change bn_expand()/bn_wexpand() to indicate failure/success via 0/1.

Change bn_expand()/bn_wexpand() to indicate failure/success via 0/1.

Currently bn_expand()/bn_wexpand() return a BIGNUM *, however none of the
callers use this (and many already treat it as a true/false value).
Change these functions to return 0 on failure and 1 on success, revising
callers that test against NULL in the process.

ok tb@