core: Fix handling of PromptForCredentials setting

core: Fix handling of PromptForCredentials setting

The prompt for credentials setting was incorrectly used in FreeRDP. If
this setting is set to 1 in a rdp file the client should prompt for
credentials even if it has credentials stored for this connection. If
the setting is set to 0 the client should either use the stored
credentials (if present) or ask for username/password otherwise.
This PR changes the old handling (if PromptForCredentials was set to 0
no credential prompting was done) to the desired behavior.