Let auth plugin handle if no username matched

[kuldeepdhaka]

mosquitto_unpwd_check_default() should return MOSQ_ERR_PLUGIN_DEFER instead MOSQ_ERR_AUTH if no username matched so that auth plugin can handle it.

According to docs: "If password_file, or acl_file are used in the config file alongsize auth_plugin, the plugin checks will run after the build in checks."

Fixes #1215

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[ralight]

This possibly needs changing, thanks for noting it. There is no way I can accept any changes to this without extensive tests though.