Adds the per-process WebRTC egress toggle + codec/payload-type knobs
described in the M2 spec. Clone() carries it forward. No behavior
change yet \u2014 the subsystem wiring comes later in M2.
1. Allow variables in placeholders for parameter values, e.g.
{rtmp,name=$processid}. The variable starts with a $ letter.
The recognized variables are provided with the Replace func.
2. The template func recieves the process config and the name of
the section where this placeholder is located, i.e. "global",
"input", or "output".
If a value for a placeholder needs escaping, add the character to
escape with an "^" to the name of the placeholder, e.g. {memfs^:}.
This will escape all occurences of ":" in the value for {memfs}
with a "\".
