Do not use word master

[pastey]

Switch to main branch instead of master in all tests and some code.
Most changes are in tests.
Minimal amount of changes in root scripts and system7 directory (the actual code). But, those changes are used only in tests :)

Reasons:

1. black lives matter
2. currently, to run integration tests on the local machine, I had to git config --global init.defaultBranch master. Ironically, GitHub runners still have some outdated git (?) and I had to add git config --global init.defaultBranch main to make tests run properly in PR checks.

Next steps:

• merge the PR
• create main branch
• set the main branch as default in this repo
• update README and How to build rd2 to the new url with install script
• set a reminder in a month to remove master branch

[nikinapi]

успешный файнд-реплейс