GUACAMOLE-1921: Process argv handler even when the connection is read-only.

[necouchman]

This simple change processes the argv handler for the VNC protocol even when the connection is read-only, which is required to prompt for credentials for read-only connections. As these are the only parameters that are processed by the argv handler, there is no additional impact on the code.

It's worth noting that, if other parameters are added in the future, the argv handler may need to check the read_only parameter and make decisions about which parameters should be allowed to be processed and which should be ignored or blocked.

[mike-jumper]

To avoid any future issues, I think we should add some comments here noting:

• That read-only may need to be considered for handling of future argument values received via argv that are not related to authentication.
• That changing things such that argv is handled for users other than the owner may have security implications unless the handling of argv is modified to only accept username/password from the owner.

Otherwise, LGTM!

[necouchman]

@mike-jumper I have added some comments - let me know if the style of those is okay, as they're a bit of a one-off in terms of the types of comments used throughout the code. In addition to the comment in user.c, I added a note to the argv.h header file where the user callback is documented.

[mike-jumper]

Very nice.