Fix transfer statistics extraction

Fix transfer statistics extraction

Make the get_named_xfer_stats() helper shell function more precise in
order to prevent it from matching the wrong lines as that may trigger
false positives for the "ixfr" and "xfer" system tests.  As an example,
the regular expression responsible for extracting the number of bytes
transmitted throughout an entire zone transfer could also match a line
containing the following string:

    transfer of '<zone-name>/IN': sending TCP message of <integer> bytes

However, such a line is not one summarizing a zone transfer.

Also simplify both get_dig_xfer_stats() and get_named_xfer_stats() by
eliminating the need for "echo" statements in them.

(cherry picked from commit fab67c074a8df8c0996c858629e9b23dff35417c)