Add ldap_sprintf() for LDAP escaping

Summary: We have some minor issues with LDAP escaping now. I think they're not security problems (they will only cause us to reject valid inputs) but add an xsprintf()-family function to fix them.

Test Plan: Did LDAP login with names like ",", got the expected errors ("bad username") instead of suspicious errors ("invalid syntax").

Reviewers: vrana, btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D2994