sout: distinguish filter and output capabilities

This removes the "sout stream" capability, and adds two new ones instead:
- "sout output" for final nodes in the sout chain, and
- "sout filter" for non-final nodes in the sout chain.

This is mainly to prevent impossible / non-working chain constructs.

Note: In practice, only "cycle" and "stats" plugins supported both modes
in any meaningful mannter. They are split into two submodules each.