Some LDAP servers allow binding with blank passwords. We should
not allow a blank password to authenticate the SSSD.

There was a faulty check in get_domain() that led to the
*_provider options being re-added, sometimes after options related
to them had already been set. If those options had a default
value, they would be overwritten by the default.

Fixes: https://fedorahosted.org/sssd/ticket/441

The PAM standard allows for messages of any length to be returned
to the client. We were discarding all messages of length greater
than 255. This patch dynamically allocates the message buffers so
we can pass the complete message.

This resolves https://fedorahosted.org/sssd/ticket/432

In the case where the allocated buffer is not large enough to hold
the resulting absolute path, we were writing out a null terminator
outside of the buffer, instead of at its beginning.

Also fixes potential issue where split_path would not initialize
the count to zero if it returned a failure.

Fixes: #81

* Do not segfault on passing NULL path to get_{dir,base}name
* There is no way dirname can return "..", remove that code
* Buffer overflow in path_concat
* Expand . in get_basename
* Return NULL rather than crash in split_path on passing NULL path
* Be more defensive in directory_list