CMake v3.5.1 throws the following error if this is not set:
```
CMake Error at channels/CMakeLists.txt:48 (if):
if given arguments:
"OFF" "OR"
Unknown arguments specified
Call Stack (most recent call first):
channels/sshagent/ChannelOptions.cmake:6 (define_channel_options)
channels/CMakeLists.txt:273 (include)
```
1. In connect_to_sshagent() if connect() fails, the socket agent_fd is
leaked. It needs to be closed before returning.
2. Fix copyright messages.
3. Make if statement with call to CreateThread() clearer to read.
Add the sshagent plugin to forward the ssh-agent protocol over an RDP
dynamic virtual channel, just as the normal ssh-agent forwards it over
an SSH channel. Add the "/ssh-agent" command line option to enable it.
Usage:
Run FreeRDP with the ssh-agent plugin enabled:
xfreerdp /ssh-agent ...
In the remote desktop session run xrdp-ssh-agent and evaluate the output
in the shell as for ssh-agent to set the required environment variables
(specifically $SSH_AUTH_SOCK):
eval "$(xrdp-ssh-agent -s)"
This is the same as for the normal ssh-agent. You would typically do
this in your Xsession or /etc/xrdp/startwm.sh.
Limitations:
1. Error checking and handling could be improved.
2. This is only tested on Linux and will only work on systems where
clients talk to the ssh-agent via Unix domain sockets. It won't
currently work on Windows but it could be ported.
