Rename git master to main

edited by nickm

Following emerging best practices (eg draft-knodel-terminology-05, current gitlab defaults), we should rename master to main. This affects the main repository so the ticket is here instead of in tor.git.

Renaming:

• Chutney
• Trunnel
• Tor
• Torspec
• Onionbalance (up to George)
• Torsocks (up to David)

Places to update:

• Gitweb (redirects may be needed)
• github CI scripts
• gitlab CI scripts
• OSS-Fuzz scripts
• nick's dodgy coverity script
• scripts/git/*
• TBB nightly build scripts?
• ... what else?