Add JackPortIsCV flag

This pull request adds a new JACK port flag, JackPortIsCV.

This is useful for applications that use JACK audio ports as CV.
As it's merely a hint, it won't hurt current JACK usage.
Patch-bays will welcome this change and may choose to represent these ports in a special way.

A new macro was also added, JACK_HAS_PORT_IS_CV_FLAG.
This allows new applications to safely check for this new flag existence.