[libhif] Exit again on SIGINT/SIGTERM
[libhif] Exit again on SIGINT/SIGTERM

A major (if not the biggest) point of this endeavor is to support
atomic/idempotent operation.

giuseppe/rpm-ostree,giuseppe/rpm-ostree,mdshuai/rpm-ostree,giuseppe/rpm-ostree,mdshuai/rpm-ostree,tmp-user/rpm-ostree,mdshuai/rpm-ostree,tmp-user/rpm-ostree,tmp-user/rpm-ostree,giuseppe/rpm-ostree,mdshuai/rpm-ostree,tmp-user/rpm-ostree