Another rounding issue

[Brian Downing]

On current master (ca42f2b9a), Ubuntu Intrepid (with libmpfr build from
the Jaunty package to get 2.4):

$ cat >rounding.dat <<EOF
2009/01/01 Sample
    assets  134.123 FOO @ $8.88
    assets  100 BAR @ $8.88
    equity
EOF
$ ./ledger -f rounding.dat print
2009/01/01 Sample
    assets                               134.123 FOO @ $8.8799999999999999975
    assets                                   100 BAR @ $8.88
    equity

-bcd