Fix random bug when workstation_name is < 6 chars

When the local workstation name is less than 6 characters, remote
authentication against a Windows 2008r2 WinRM service always fails. This
doesn't seem to affect authentication against IIS's negotiate
implementation.
Land #3363, Workstation Length Auth Bug
Note about EXE::Custom
Add a small note to bypassuac_injection concerning EXE::Custom