umoci-insert needs to support (opaque) whiteouts

[cyphar]

Basically it should be possible to do both of these:

% umoci insert --opaque ./some/dir /etc # mask old /etc/* entries
% umoci insert --whiteout /opt/blah     # delete this entry

The former should mask away all of the previous entries under /etc (by adding an /etc/.wh..opq to the layer), while the latter should create a whiteout of the path /opt/blah. This is necessary for a generally-usable insertion system.

Also we should consider how this will interact with the long-term goal of #256.