BUG/MAJOR: ssl: OpenSSL context is stored in non-reserved memory slot

BUG/MAJOR: ssl: OpenSSL context is stored in non-reserved memory slot

We never saw unexplicated crash with SSL, so I suppose that we are
luck, or the slot 0 is always reserved. Anyway the usage of the macro
SSL_get_app_data() and SSL_set_app_data() seem wrong. This patch change
the deprecated functions SSL_get_app_data() and SSL_set_app_data()
by the new functions SSL_get_ex_data() and SSL_set_ex_data(), and
it reserves the slot in the SSL memory space.

For information, this is the two declaration which seems wrong or
incomplete in the OpenSSL ssl.h file. We can see the usage of the
slot 0 whoch is hardcoded, but never reserved.

   #define SSL_set_app_data(s,arg)     (SSL_set_ex_data(s,0,(char *)arg))
   #define SSL_get_app_data(s)      (SSL_get_ex_data(s,0))

This patch must be backported at least in 1.8, maybe in other versions.