- Instead of taking the first new thread as an out-parameter, we now
bundle the process and its first thread in a struct and use that
as the return value.
- Make all Process factory functions return ErrorOr. Use this to convert
some places to more TRY().
- Drop the "try_" prefix on Process factory functions.
There was only one permanent storage location for these: as a member
in the Mount class.
That member is never modified after Mount initialization, so we don't
need to worry about races there.
