Make sure int SSL_COMP_add_compression_method() checks if a certain

Make sure int SSL_COMP_add_compression_method() checks if a certain
compression identity is already present among the registered
compression methods, and if so, reject the addition request.

Declare SSL_COMP_get_compression_method() so it can be used properly.

Change ssltest.c so it checks what compression methods are available
and enumerates them.  As a side-effect, built-in compression methods
will be automagically loaded that way.  Additionally, change the
identities for ZLIB and RLE to be conformant to
draft-ietf-tls-compression-05.txt.

Finally, make update.

Next on my list: have the built-in compression methods added
"automatically" instead of requiring that the author call
SSL_COMP_add_compression_method() or
SSL_COMP_get_compression_methods().