LP#1282751 Credit card payment balance owed rounding fix

LP#1282751 Credit card payment balance owed rounding fix

Avoid using Perl's int() when summing owed/paid totals for display in
the TPAC credit card payment form, since this can lead to rounding
errors.

A simple example of why we should not use int() when summing floating
point numbers:

perl -e 'print "no match\n" unless ((8.29 * 100) == int(8.29 * 100))';

Furthermore, use the relatively new fpsum() utility function for summing
floating point numbers so we can avoid having multiple versions of the
summing logic floating (*cough*) around (*cough cough*).

Signed-off-by: Bill Erickson <berickxx@gmail.com>