patches for openconnect

[Nikos Mavrogiannopoulos]

I realized my previous patches for openconnect no longer apply in master.

I've rebased them and placed them at github:
https://github.com/nmav/openconnect-mine/commits/master

commit 89a62ecf67322c3aada8ead213ca190aa87d91c3
    Added chacha20-poly1305 as a DTLS ciphersuite for gnutls
commit 334c4e2b63b1d7e71f34185b1069192c050ab076
    only enable the DTLS ciphersuites that match the ones enabled in TLS
    That currently is restricted to gnutls code.
commit 57c9add5b41bef48bdf2c09f30f016dd74d722b2
    Allow overriding the default GnuTLS priority string
commit 26ffcbb707bee39d0d20c92d4e5217fd013e0abd
    Static checks for gnutls version were made dynamic

regards,
Nikos