e13b602 ("Avoid using pthread_cancel") introduced a bogus termination test,
this fixes it.

Fixes #572

If a thread is cancelled while it holds a mutex, other threads waiting for
the mutex will never be woken, even if destroying the mutex, thus
leading to a deadlock.

Better just make the thread loops have a proper exit case than risking
the tricky semantic of pthread_cancel.

This reverts commit 6042eb8d1e.

The change causes an issue with `pt-BR` and possibly other languages
and voices.

Fixes #555.

The replacement tests for bs, hr, and sr are no longer marked as
broken as they work using the old code. The mk tests keep the
broken annotation, as they don't work in the old code either.

This reverts commit 801a8d197c.
This reverts commit 64d5701e5e.
This reverts commit 3b51ebf617.
This reverts commit 1fd235d2c0.
This reverts commit 9f0667de86.

PrepareLetters() function also could be used to set up letter groups A, B, C, etc. from string
and further could be loaded from configuration file

Fixes #511.

When .replace rule is used and -X switch is passed in runtime,
replaced characters should be shown in trace output.