client authenticate callback

[awakecoding]

Add an interactive password prompt when connecting without passing enough of the credentials at command line.

[awakecoding]

Currently, xfreerdp will prompt for a password if it wasn't passed at command-line first. However, to complete this task, we should have the core make use of callback to have the UI prompt the user for its credentials.

[shlevy]

@awakecoding: Should I work on this? I'm not sure exactly what this would entail, what should behave differently than it does currently?

[awakecoding]

@shlevy: this one should be a no-brainer to me, it's more a change in the code structure than in functionality. we need to have the core make use of a callback similar to those used in graphical updates, and have the UI register for it instead. This would allow for clients like wfreerdp to use a graphical prompt instead of the command-line version, for instance.

[awakecoding]

client authenticate callback has been added, some polishing might be required though, such as asking for the username and password if it hasn't been entered as well