UTIL: Inherit parent domain's default_shell

Some override parameters were not inherited when creating subdomains.
Especially with AD trusts, this gave strange results.
NSS: Use plain user name when expanding homedir
simple access: match objects using flat name

Use flat name to recognise users and groups belonging to main sssd domain.

Resolves:
https://fedorahosted.org/sssd/ticket/2189
simple access: refresh master domain info

To correctly decide if an object is a member of the main sssd domain, a flat name
is needed. However, the information may not be available when the module is
inited so it may be necessary to refresh this data later while processing a
request.

Resolves:
https://fedorahosted.org/sssd/ticket/2189
NSS: add support for subdomain_homedir

Resolves:
https://fedorahosted.org/sssd/ticket/2169
Teach PAM client to do multi-pass

First cut at multi-pass in the PAM module.  We bump the version to 4 and
add a request item that contains both a context ID and a subcommand.

By default, that subcommand is "one-shot", indicating the traditional
behavior where the responder hands us a final reply.

When passed "multi_pass" as an argument, the module changes that
subcommand to "start".  So long as the reply comes back with status
INCOMPLETE and a substatus of to-be-continued (with a matching context
ID, of course), we parse out any AUTH_REQUEST items in the reply, select
a group of them to be attempted, collect answers for them, and submit
another request with the same context ID, a subcommand of "continue",
and their values as AUTH_ANSWERs.
Expand sss_authtok to handle secrets, OTPs, PINs
Display the type of PAM auth tokens when debugging
Rework item formats to meet lengths-first expectations
pam_sss: tag authtok items
pam responder: parse and pass along multistep info
Pass authtoks around; pass multi-step data to dp's
Flesh out debug messages
Handle all the prompts
Make it easier to add prompts
Add a password prompt type
Add the rest of the prompt types
Add a multi_step_continue_expected field