[ofiwg] libfabric v1.19.0 rc 1

[Hefty, Sean]

I created v1.19.0rc1 and pushed the tar files up to github:

https://github.com/ofiwg/libfabric/releases/tag/v1.19.0.rc1

Going forward, changes that target v1.19.0 will need to be cherry picked from main into the v1.19.x branch that was also created.  I anticipate the final v1.19.0 release within 2-4 weeks.

I am also planning on a v1.18.2 release soon after, to pick up fixes which missed v1.18.1 or are discovered during the v1.19.0 release cycle.

- Sean