Per the discussion on sssd-devel list, nss_sss should not return a
hardcoded value but this should rather be configurable to allow whatever
the OS or distribution thinks is the best for the particular case.

Fixes: #266

The providers are now responsible for determining how long a cached
entry is considered valid. The default is the same as before (600s)

This adds a new option (offline_credentials_expiration) to the
[PAM] section of the sssd.conf

If the user does not perform an online authentication within the
timeout (in days), they will be denied auth once the timeout
passes.

Create and populate user directories on useradd, delete them on userdel

Fixes: #212

The LOCAL provider does not have a mechanism to load alternate
auth, access or chpass backends, nor does it make sense to do so.

This will throw a configuration error if these values are
specified (unless they are explicitly also set to 'local')

In sssd only local is a native mpg domain, and it is forced.
All other providers will have to unroll mpg users into a user/group pair of
entries in the db. This allows the provider to automatically establish if
the remote server provides mpg users w/o possibily conflicting manual
configurations on the client trying to force an mpg behavior where none
is provided.

We have converted to using dhash in place of btreemap everywhere
in the code.