ivfenc was setting the VPX_KF_FIXED mode when the kf_min_dist and
kf_max_dist parameters were set to each other. This flag actually means
that keyframes are disabled, and that name was deprecated to avoid
confusion such as this. Instead, a new option is exposed for setting the
VPX_KF_DISABLED mode, and the intervals are passed through to the codec,
which will do automatic placement at a fixed interval as expected.

Change-Id: I15abbec5936f39d5901878b4bc154372fbc23a43