Vendor import of Gavin Howard's bc version 3.2.6

(cherry picked from commit 47a52dc4d48f259ab7d9f9ba6b65f4f2331a22dc)

Second attempt after revert of a previous commit that affected files
with similar names in unrelated directories.

Thanks go to Martin Birgmaier for reporting the issue and identifying
the cause of git getting confused: files from the root of the vendor
files were associated with files in the FreeBSD base root directory.

Martin Birgmaier points out that to make this cherry-pick succeed,
the option -Xsubtree=contrib/bc has to be passed to prevent files
outside that path to be considered.