Send 'cas-support=yes' in GlobalProtect prelogin request

Send 'cas-support=yes' in GlobalProtect prelogin request

Per https://gitlab.com/openconnect/openconnect/-/issues/651, some newer GP
servers are responding to prelogin.esp requests with an error:

    CAS is not supported by the client.  Minimum client version is 6.0

It appears that CAS ("Central Authentication Server";
https://apereo.github.io/cas/index.html) is a standardized single-sign-on
protocol requiring an external browser.

Per https://gitlab.com/openconnect/openconnect/-/issues/651#note_1576596243,
the field 'cas-support=yes' needs to be sent in the POST *body* of the
prelogin request, in order to avoid this error message; the error message's
claim that a specific client software version is necessary isn't very
helpful.

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