This adds a new build tag named "safe" which serves the exact same
purpose as the current "disableunsafe" tag.  This is being done, as
recommended by @shurcooL, since it is emerging as the standard way to do
it in several high profile packages, it mirrors the "unsafe" package
nicely, it is shorter, and users generally seem to prefer it.

However, to avoid breaking existing infrastructure, the disableunsafe
tag is still available and simply is being deprecated.