Release Candidate v3.9.4.0-rc1

[Jeff Long]

Here is a new release candidate, somewhat earlier than expected, due to a couple of regressions in v3.9.3.0

https://github.com/gnuradio/gnuradio/releases/tag/v3.9.4.0-rc1

Some other fixes and features were also backported in the process, for more details, see

https://github.com/gnuradio/gnuradio/blob/v3.9.4.0-rc1/CHANGELOG.md

The final v3.9.4.0 will be out next week if there are no issues, and the next set of releases (3.8.x.y and 3.9.x.y) will be out around December.