create_user_session support for whostmgrd

Developing for account related security could see benefits in implementing different service types for this API function, not just limited to whostmgrd(WHM) - but also webmail.

http://docs.cpanel.net/twiki/bin/view/SoftwareDevelopmentKit/CreateUsersession

The function already allows for an input variable of 'service', initially I presumed that all service types were supported (as it is is not explicitly stated otherwise) - so I presume that this is an intended feature for the future anyway.

The benefits to this would be custom control panel related:

- Removing the need to share root passwords with users

- Password-less logins, using a simple unique identifier instead of passing through sensitive information

It's nice to being able to 301 redirect users into their cPanel account, why not have the same for WHM? I realise this would be up to the developers discretion to lock down these capabilities, but a non-competent developer typically would not/should not be developing with these functions anyway.