contrib/bc: merge version 5.1.0 from vendor branch

This version adds options and functions that allow to print numbers
in the open interval (-1 .. 1) with or without a leading 0 digit.

Additionally, an option has been added to prevent line wrap and
allows to print arbitrarily long results on a single line.

Merge commit '5d58a51571721190681c50d4bd3a1f45e6282d72'