Make bitcoin_my_transactions_signing_keys_p() actually attempt signing

Rather than trying to do our own privkey accounting (which is going
to break as soon as we allow the user to specify privkeys which
might not have specific inputs associated to them), we simply sign
the transaction and check if it's complete. bitcoind kindly gives
us the complete field in the response of signrawtransaction, so this
is easy :).
settings: trim setting values properly

goodbtc is having some parse troubles which (I think) were due
to the blind "subtract 1 from the length" method of chopping
off trailing whitespace in setting values. Now we actually scan
from beginning and end to clear off any and all whitespace.