(ITS#6147) slapd should explicitly configure SASL auxprop

[hyc@OpenLDAP.org]

Full_Name: Howard Chu
Version: 2.4.16/HEAD
OS: Linux
URL: ftp://ftp.openldap.org/incoming/
Submission from: (NULL) (80.67.104.110)
Submitted by: hyc


we need a config option for controlling which auxprop plugins the Cyrus SASL
library uses, and it should default to just "slapd". 

See ITS#6056, the sasldb plugin causes problems and generally isn't useful
anyway. (And yes, we probably need to investigate why those problems arise.)

Also see ITS#6129, we should take more explicit steps to prevent the ldapdb
auxprop from loading.