Convert all the checks in repoman.checks to use a new LineCheck

Convert all the checks in repoman.checks to use a new LineCheck
interface that takes a single line as an argument. This has at
least a few of benefits:

 * Eliminates lots of redundant code
 * Error messages are ordered by line number across all checks

The performance is slightly worse due to the increased number
of method calls, but it's not really noticeable in comparison
to the time consumed by dependency checks.

Thanks to Petteri Räty <betelgeuse@gentoo.org> for the initial
patch which I only made a few minor modifications to.

svn path=/main/trunk/; revision=8483