Is now intended to the used for user deinitialization.
This fixes mail_user_session_finished event not being sent
with managesieve.

Also change the callers to not bother storing it separately. Instead, they
can just use mail_user_r->service_user.

This also drops the trailing period from the log message, which matches the
changes to core.

This makes more sense with the managesieve_command_name field.

Also dropped the single-letter identifier for most fields to prevent conflicts with mail user variables; these were undocumented anyway.

Cleanup performed with the following semantic patch:

	@@
	expression E;
	@@

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

Cleanup performed with the following semantic patch:

	@@
	expression E;
	@@

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

	@@
	expression E;
	@@

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

Mirrors imap fix in Dovecot: 42149f48624b82fdf9631c256497580154c2e412

It's impossible for the command's pointer to be NULL at this point.
Previously, the command_find() would have returned NULL, but this check
presumably short-circuits that search in the trivial case, so has some
real use.

Problem now found by GCC 7.