Currently `IsTraced("xyz")` will return true for
any inclusion of "xyz" in string.
This change splits `STTRACE` using `','`, `' '` and `';'`
as delimiters. That makes facilities separation
more clear.
### Purpose
Instead of hardcoding `SigningKey` as text use `go:embed`. Fixes#9247.
### Testing
* Building syncthing
* Trying to upgrade (signature verification)
gudvinr