frontend: Allow NLA auth for Native RDP execution

This patch allows Network Level Authentication in combination with
Native RDP client. This is done by setting enablecredsspsupport flag in
RDP descriptor to 1.

Previously this flag was set to 0 which prevented clients to connect to
RDP server with NLA enabled.

This patch only affects 'Native client' invocation. The 'Browser plugin'
still has this option disabled (enabling it would prevent passing
password to ActiveX plugin and therefore SSO would be impossible).

Also RDP info icon text in 'Console options' dialog was enhanced with
information about NLA and SSO.

Change-Id: If686b824aa05b6d0083b9be6eca681797c3c26f3
Bug-Url: https://bugzilla.redhat.com/1131693
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>