GlobalProtect: apparently, the parameter `clientos=Linux` value is not just allowed...

GlobalProtect: apparently, the parameter `clientos=Linux` value is not just allowed, but necessary, for some VPNs.

* Previously, I had received at least two reports of servers where
  `clientos=Windows` was required for the VPN to work correctly.

* Per https://github.com/dlenski/openconnect/issues/126, there is at least
  one report where *not* setting `clientos=Windows` was required for the VPN
  to work.

* The truly maddening part is not only the pointless and inconsistent
  behavior of the GlobalProtect servers, but also the fact that the servers
  give such misleading and irrelevant error messages ("Incorrect username or
  password" or "Unable to assign private IP address", etc.) rather than
  something that makes sense like "Unknown clientos value."

This patch makes `clientos=Linux` the default behavior when
`vpninfo->platname` is `linux-64` or `android`, while still allowing it to
be overridden with `--os=win` etc.

Signed-off-by: Daniel Lenski <dlenski@gmail.com>