Fix help text issues across cmd/desync subcommands (#332)

- mount-index: replace non-existent `-x` in example with `--cor-file`
- extract: correct `-seed`/`-seed-dir` to `--seed`/`--seed-dir` in description
- inspect-chunks, info: align second example line indent with first
- chunk-server, index-server: rename `--writeable` flag to `--writable`,
  keep `--writeable` as a deprecated alias for backward compatibility
- inspect-chunks: rename `newinspectChunksCommand` to `newInspectChunksCommand`
- root: drop trailing period from Short for consistency
- untar: add missing article in Short ("Extract a directory tree ...")
- cache: fix awkward `index(s)` in --ignore flag help

Do not call t.Fatal() from non-main goroutines (#291)

Calling t.Fatal() only stops the current goroutine, but not the rest.
The result is different depending on a test, but in the worst case
scenario (like an error in the server part of a test) the client may
hang waiting indefinitely for the response, and the test ends with
a panic from the test timeout, without showing the real reason for
the issue.