please rename mptcpd 'master' branch to 'main'

[dcaratti]

many open-source projects are gradually transitioning their 'master' branch to a less inconvenient name ('main' or similar) [1] [2]. Any objections to transition mptcpd as well?

(BTW, I see that the kernel code can be made 'compliant' with a 3-line patch, I can follow-up on the ML if you don't disagree)

[1] https://lore.kernel.org/git/pull.656.v4.git.1593009996.gitgitgadget@gmail.com/ [2] https://sfconservancy.org/news/2020/jun/23/gitbranchname/

[mjmartineau]

I've renamed the default branch to main (to update references in github PRs and such).

For now, master is still pointing to the same git commit. After a transition period we can remove that reference.

[ossama-othman]

Thanks for suggesting and making this change!