managesieve: Managesieve used "managesieve" rather than "sieve" as login service name, which means that all managesieve-specific settings where ignored.

Most important change is that mail service_user was not freed.

This change matches a similar change in Dovecot for IMAP/POP3.
Also includes other small changes that make ManageSieve match IMAP implementation a little more closely.

"Running standalone?" check now uses a new DOVECOT_CHILD_PROCESS environment rather than GENERATION.
The GENERATION environment was already set in some systems for Java.