Fix dpkg-buildpackage to not double options when called with -j option

Fix dpkg-buildpackage to not double options when called with -j option

* scripts/Dpkg/BuildOptions.pm (set): Change the default value of
the overwrite parameter to 1. The only use of this function in
dpkg-buildpackage is missing the parameter and actually wants to
overwrite the whole variable.
* scripts/t/300_Dpkg_BuildOptions.t: Adjust the test suite for this
change.