Add 'refresh' option to '-M'.

aura -My will now refresh the ABS tree. Additional options may be
carried out after refreshing the tree.

At present, the option to do a partial sync of the ABS tree has been
removed, though maybe this should be added back. Does this potentially
result in inconsistencies?
Implement 'ShowPkgBuild' option to ABS (-Mp).
Implement dependency display for ABS (-Md).

Also don't fail if the package doesn't have a description.
... but do write the 'no description' message in red.
Implement function to enable 'absSyncLocal'.
This will try to synchronise only the existing abs tree on a refresh.

However, I haven't enabled this as default or alternative functionality
yet, because I think it should possibly be more clever in terms of
deciding when to forget about syncing individual packages and just
fetch the whole tree, or specific repositories. Currently, a 'local'
sync on an existing tree will build up an extremely large command.