encode: get rid of AVDictionary setter helper

Removes a good hunk of weird code.

This loses qscale "emulation", some logging, and the fact that duplicate
keys for values starting with +/- were added with AV_DICT_APPEND. I
don't assign those any importance, even if they are user-visible
changes.

The new M_OPT_ flag is just so that nothing weird happens for other
key-value options, which do not interpret a "help" key specially.