Start adding LDAP back-end. Requires change to all back-ends b/c LDAP needs to see the password and authenticates at the directory server (instead of returning PBKDF2)
start on LDAP
cleanup
new connection for authenticating user's DN
	sundry cleanups
clear authenticated_be on alloc, and add warning if oob for #20
add debug for mosquitto_auth_acl_check() to prove invocation
nit
docs for LDAP