back-sch-nss: for users with aliases, return alias as uid
When SSSD resolves AD users on behalf of slapi-nis, it can accept
any user identifier, including user principal name (UPN) which
may be different than the canonical user name which SSSD returns.
As result, the entry created by slapi-nis will be using canonical user
name but the filter for search will refer to the original (aliased)
name. The search will not match the newly created entry.
Fix this issue by returning two values for 'uid' attribute: the
canonical one and the aliased one. This way search will match.
Verified that SSSD with id_provider=ldap happily consumes such entries.
By LDAP schema, 'uid' attribute can have multiple values.
Fixes https://fedorahosted.org/slapi-nis/ticket/12