consistently use ten Xs in mkstemp(3) templates patch from Josiah Frentsos, thanks!

add gotadmin init -b <branch> to specify repo head ref Similar to `git init -b`. Includes a change to `got import` behaviour such that "main" is no longer hardcoded by default; instead, we import to the branch resolved via the repository's HEAD reference unless `got import -b` is used, and only if HEAD cannot be resolved to a branch do we fallback to "main". includes fix plus ok from stsp@