Previously, sss_useradd defaults were hardcoded with no way to
change user's default shell or base for home directory. This patch moves
them into config/user_defaults

Convert auth modules to do the caching themselves

Fixes: RHBZ #498462

Make shadow-utils base path configurable
Use default values for params, allow configuring them

- added range check for supplied UIDs and GIDs
- initialize pc_gid to 0 to trigger gid generation

Change sysdb to always passwd sss_domain_info, not just the domain name.
This way domain specific options can always be honored at the db level.

To be able to correctly filter out duplicate names when multiple non-fully
qualified domains are in use we need to be able to specify the domains order.

This is now accomplished by the configuration paramets 'domains' in the
config/domains entry. 'domains' is a comma separated list of domain names.
This paramter allows also to have disbaled domains in the configuration without
requiring to completely delete them.

The domains list is now kept in a linked list of sss_domain_info objects.
The first domain is also the "default" domain.

Gecos, homedir and shell are optional, fix the responder not to refuse to return
the user completely if they are missing, replace an empty homedir with "/".

Also fix fullname vs gecos, and always return gecos for NSS data.
On user creation set gecos to the same value as the user Full Name, to help
populate the gecos field with data that makes sense.