pt-table-checksum can crash with --columns if none match

If pt-table-checksum is given the --columns option, it can crash:

$ pt-table-checksum --tables mysql.user --columns foo
06-21T12:03:51 Error checksumming table mysql.user: Use of uninitialized value in concatenation (.) or string at /home/baron/2.1/bin/pt-table-checksum line 3200.

            TS ERRORS DIFFS ROWS CHUNKS SKIPPED TIME TABLE
06-21T12:03:51 1 0 0 0 0 0.000 mysql.user

This is because the behavior of --columns is not defined well enough. To fix this, we should modify the tool so that:

1. The documentation states that a --columns list will select all matching columns from the table. If any specified columns don't exist, it is not an error. That is, the columns to be checksummed are an intersection of the columns in the table, and the columns mentioned in --columns.

2. If no columns are to be checksummed, the table should be skipped with this message: "Skipping $table because all columns are excluded by --columns or --ignore-columns."

3. Ensure that the same is true if --ignore-columns filters out all columns.

4. Check other tools that may have --columns or --ignore-columns options also.