Hash functions for custom-safe-themes

[Chong Yidong]

When we added custom-safe-themes about a year ago, we used SHA1 for
hashing, just because that was conveniently implemented by `sha1.el'.
Several months later, we got built-in SHA256 hashing on all platforms
thanks to Gnulib and the secure-hash primitive.

For future proofing, maybe it would be wise to switch to SHA256 for
custom-safe-themes, before the release.  Any thoughts?