If RESTRICT_USER is empty string, don't call initgroups().
If RESTRICT_USER is empty string, don't call initgroups().

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branch : HEAD

jkerihuel/dovecot,jwm/dovecot-notmuch,jkerihuel/dovecot,jwm/dovecot-notmuch,jwm/dovecot-notmuch,jwm/dovecot-notmuch,jkerihuel/dovecot,jkerihuel/dovecot,jwm/dovecot-notmuch,jkerihuel/dovecot