[ofiwg] renaming libfabric master branch to main

[Hefty, Sean]

There have been a few request to rename the libfabric master branch to main, with discussion in the ofiwg.  Github provides tools to help assist with the rename, and in theory the migration should be painless for most users.

I plan on handling the rename early tomorrow (Tuesday the 8th), so any problems can be responded to.

Open PRs should update automatically.  Local repos can be updated using the following sequence of commands:

git branch -m master main
git fetch origin
git branch -u origin/main main

- Sean