Add package removing function.

Add ability to remove a package from repository.

Packages can now be forced added to a repository by specifying
force flag. This is necessary to incremental update, as in some
circumstances we *want* to downgrade repository (for example,
a rollback to stable branch).
Add ability to load manifests from FILE.

Loading yaml from a file stream seems to be a good idea, as we
operate with files objects literally everywhere.

Validate packages loaded from manifests.

Use sbuf_len to assume that string is not of zero size.