Problem: When calling an API from Vimscript to set an option, mapping,
etc., :verbose shows that it's set from an API client.
Solution: Don't override current_sctx.sc_sid when calling an API from
Vimscript. Also fix the inverse case where API channel id is
not set when calling an API from RPC. Move channel id into
sctx_T to make saving and restoring easier.
Related #8329