#105 support for GnuTLS

[Daniel-Constantin Mierla]

Upload of the work done by Juline Blache to add support for GnuTLS.

Here's the diff:
- Makefile: use the gnutls config helpers
- cfg.y: drop old & deprecated SSLv2 option
- tls/tls_config.h: remove unused enum values
- tls/tls_domain.h: prepare for gnutls
- tls/tls_init.c: partial gnutls conversion
- tls/tls_init.h: prepare for gnutls
- tls/tls_server.c: prepare for gnutls

Where to go from here:
- need to define an openser replacement for SSL_CTX; SSL_CTX as
provided by the gnutls openssl compat layer is a good starting
point (ditto for other openssl structs that need to be replaced)
- SSL options needs to be checked/redefined wrt gnutls options and
openssl options
- do we need to explicitly set the cipher list ? The cipher priority
should be enough.

[Daniel-Constantin Mierla]

[Daniel-Constantin Mierla]

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It is partial work, should be continued (cannot be used as it is now).

[Olle E. Johansson]

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Does GNUtls support external SSL acceleration boards like OpenSSL?