dovecot: allow sasl_bind=yes in the LDAP driver.

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).