Makes code easier on the eyes.

For consistency with managesieve.

It's not safe to use io_add() with SSL connections.

This commit removes the previously deprecated and now fully dropped
src_pos_r parameter from the base64_decode() function calls.

This allows passing passdb variables. They are prefixed with forward_ when
imported to extra fields.

client_destroy() already adds it.

Previous commit was accidentally for an earlier version of the API change.

Also explicitly initialize login_binary->anonymous_login_acceptable to FALSE to
make clear that anonymous logins are intentionally not allowed for ManageSieve.

Cleanup performed with the following semantic patch:

	@@
	expression E;
	@@

	- if (E != NULL) {
	- 	timeout_remove(&E);
	- }
	+ timeout_remove(&E);

Commands that continued after waiting for more input always failed.
This mainly applies to the AUTHENTICATE command, which mostly handles its own arguments.
This caused problems when a string literal was used as the initial response for the AUTHENTICATE command.

Caused by recent change:

Author: Timo Sirainen <timo.sirainen@dovecot.fi>
Date:   Wed Jun 14 12:39:02 2017 +0300

managesieve-login: managesieve_client_input_next_cmd() - minor code cleanup