There is no description for the local 'curl' USE flag for sys-apps/systemd yet. Please add this to metadata.xml: <flag name='curl'>Enable support for uploading journals and import of container images</flag>
+ 18 Jan 2015; Michał Górny <mgorny@gentoo.org> metadata.xml: + Describe USE=curl, bug #536934. Thanks to Elias Probst for the description. Thanks a lot!