index+xcpdb: rename `--no-sync' to `--no-fsync'

index+xcpdb: rename `--no-sync' to `--no-fsync'

We'll continue supporting `--no-sync' even if its yet-to-make it
it into a release, but the term `sync' is overloaded in our
codebase which may be confusing to new hackers and users.

None of our our code nor dependencies issue the sync(2) syscall,
either, only fsync(2) and fdatasync(2).