End transactions before erroring out

If we don't end the transaction properly on error, we'd leave the VCC line
enabled when attempt to program via ICSP programming attempt fails the
Chip ID check.
Add -v option

Possibility to skip verification.
Exit upon encountering unknown options
Propagate Config_open() errors more sensibly
Fix the line length check

The condition used to be always true as fgets chops off the extra bits
(thankfully!).
Propagate errors from Config_set_*()
Add bounds checks to configuration parser

Eek!
Deal with missing fuse file keys gracefully
Merge pull request #27 from lkundrak/errors

Some random bug fixes