Support downloading from local repository even without libcurl

Using the --url option of the razor executable, it is possible
to specify a yum repository on the local machine (eg., on installation
media) and import from there, eg.,:

C> razor --url file:///d:/ import-yum

This will be handled by libcurl if available but if not, an internal
copy routine will be used.

Note that if Microsoft's KTM implementation of atomic transactions is
used, then the current directory must support atomic transactions
(also improve error messages for this, and other, cases).