Add opam option require-checksum

[rjbou]

For the moment, to require or disable checksum checks there is 2 ways: options --require-checksum and --no-checksum, and their respective environment variable OPAMREQUIRECHECKSUMS and OPAMNOCHECKSUMS.
For permanent configuration, it is better to have a field in global config that can be modified via opam option instead of environment variable.