Dovecot has its own SASL implementation,
but needs Cyrus SASL's headers to bind to an LDAP server using SASL.
This is useful to avoid the need to manage a dnpass= in dovecot-ldap.conf
by using the Unix socket to authenticate.
This is done with sasl_mech=EXTERNAL in dovecot-ldap.conf, and some olcAccess: with
by dn="gidNumber=0+uidNumber=0,cn=peercred,cn=external,cn=auth" read
in the slapd's cn=config for the LDAP database queried by dovecot/auth (which runs as root).
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