Instead of marking unreachable code with `ZEND_ASSERT(0)`, we introduce
`ZEND_UNREACHABLE()`, so that MSVC which does not consider `assert(0)`
to mark unreachable code does no longer trigger C4715[1] warnings in
debug builds.  This may be useful for other compilers as well.

[1] <https://docs.microsoft.com/de-de/cpp/error-messages/compiler-warnings/compiler-warning-level-1-c4715?view=vs-2019>

The implementation of that RFC changed the initial value of
`zend_array.nNextFreeElement` to `-1`; we work around that by inserting
first, and retrieving the index afterwards.

We also fix the erroneous printf specifier for the unsigned integer.

* PHP-7.4:
  Fix #73927: phpdbg fails with windows error prompt at "watch array"

We expect zvals, so we should request zvals.

We also suppress spurious watchpoint removal notices.

* PHP-7.4:
  Enable phpdbg tests on AppVeyor
  Make phpdbg test portable
  Fix several mostly Windows related phpdbg bugs
  Fix #73926: phpdbg will not accept input on restart execution

* Properly initialize PHPDBG_G(watch_tmp)

  Otherwise that may cause segfaults in ZTS builds.

* Deactivate potentially remaining watchpoints after REPL

  Otherwise the memory could still be protected, resulting in segfaults
  during shutdown.

* NULL zend_handlers_table after freeing

  As of commit 4130fe4[1], the `zend_handlers_table` is explicitly
  freed in the `zend_vm_dtor()`.  Since phpdbg (and maybe some other
  SAPIs) may restart the engine afterwards, we have to make sure that
  the table is also NULLed.

* Only set context option if there is a context

  In other words, we must not follow the null pointer.

* Cater to file handles without attached console

  File handles do not necessarily have an attached console (for
  instance, pipes do not), in which case `GetConsoleScreenBufferInfo()`
  fails.  In this case we set a default value (`40`) for lines like on
  other systems.

[1] <http://git.php.net/?p=php-src.git;a=commit;h=4130fe437a5db7ead1444d3748bd0fbad9829cb2>

Closes GH-4732.