rename config to annex.security.allowed-compute-programs

rename config to annex.security.allowed-compute-programs

And require for enable as well as autoenable.

It seemed asking for trouble for `git-annex enable foo` to use whatever
compute program is stored in the git config, without verifying that the
user wants that program to be used.

Note that it would be good to allow `git-annex enable foo program=...`
to be used without the program being in the git config. Not implemented yet
though.