openconnect MTU detection fixes

[Nikos Mavrogiannopoulos]

Hi,
 I noticed while debugging ocserv that openconnect only sets the
X-CSTP-MTU header, even though there is code to set the
X-CSTP-Base-MTU. The latter is more useful header for the server as it
informs about the link MTU between the client and server, rather than
an estimation of the tunnel MTU that _will_ be setup.

The issue seems to be two uninitialized variables and a missing
header, however, with the attached patches I went further improving
these calculations, and always setting base_mtu since this value
provides the server with more information to act in selecting the MTU
(the X-CSTP-MTU is a value which depends on the selected DTLS cipher
parameters and is hard to work with).

regards,
Nikos
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