MDEV-34814 mysqld hangs on startup when --init-file target does not e…

[DaveGosselin-MariaDB]

…xist

Ensure that select_thread_in_use is cleared before calling unireg_abort(), otherwise when unireg_abort() calls wait_for_signal_thread_to_end(), it will block indefinitely. Factored this common pattern in mysqld_main to a helper function to ensure this is done uniformly.

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