support for user-redefinable actions

[UnixJunkie]

Hi,

I'd like to do the following work:
I will introduce actions in odb.
Actions on each package are: get | configure | build | install | uninstall

Then, the current default for each of these steps as currently
being done by odb will become an action (code re-factoring with this new type).

Then, I will provide via the packages file the possibility for users
to redefine the default for any of these actions for each package.

This is for developer and advanced users of odb.
Standard users won't have to care about this and the default
will continue to work, also the new keys for the packages file
won't be mandatory.

Regards,
F.