wlfreerdp: validate the TLS certificate interactively

Allow the user to decide if he trusts a new certificate
or not, just like we do with xfreerdp ; this allow running
wlfreerdp without using xfreerdp prior for this sole task.

Signed-off-by: Manuel Bachmann <tarnyko@tarnyko.net>
Merge pull request #2202 from Tarnyko/master

wlfreerdp: validate the TLS certificate interactively