net/freerdp: update to 2.1.0

This update incorporates many features and improvements since 2.0.0-rc4,
as well as a large mass of security fixes.

Full changelog available:
https://github.com/FreeRDP/FreeRDP/blob/2.1.0/ChangeLog

PR: 245517
Approved by: koobs (mentor)
MFH: 2020Q2
Security: 669f3fe8-a07a-11ea-b83e-f0def1f5c5a2