rename master branch to main

[Oliver Beckstein]

master is a word with many negative connotations, at least in US history. Given alternatives such as main (which is becoming the de-facto standard), switch over.

See https://github.com/github/renaming

[Oliver Beckstein]

For people with a local copy of master:

git branch -m master main
git fetch origin
git branch -u origin/main main
git remote set-head origin -a

[Oliver Beckstein]

renamed