Fix some more ldap issues

Summary:

• LDAP import needs to use envelopes.
• Use ldap_sprintf().

Test Plan: Configured an LDAP server. Added an account. Imported it; logged in with it. Tried to login with accounts like ",", etc., got good errors.

Reviewers: vrana, btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D2995