movenc: Add a flag for using default-base-is-moof in tfhd atoms

Similarly to the omit_tfhd_offset flag added in e7bf085b, this
avoids writing absolute byte positions to the file, making them
more easily streamable.

This is a new feature from 14496-12:2012, so application support
isn't necessarily too widespread yet (support for it in libav was
added in 20f95f21 in July 2014).

Signed-off-by: Martin Storsjö <martin@martin.st>