"Hack": LDAP lookup support to convert "usernames" to "mail" attribute.
E.g.:
LDAP entry: username=gdestuynder, mail=gdestuynder@mozilla.com
Unix entry: gdestuynder:x:1000:....
DuoSecurity Account: gdestuynder@mozilla.com

When trying to login via pam_duo, we will lookup the matching mail=
entry for the username and authenticate to duo with that. Especially
useful if the username is completerely different from the mail/account
entry.

Additional notes (if fixed, could even stop calling this a hack.):
- uses LDAP_DEPRECATED
- Some LDAP values appears not to free correctly, althought being a separate
  process there should be no real memleak. See comments in code.
- Config options are hardcoded. Should move them to a conf file
  eventually.