Allow hardcoding of min/max minor/major SSL version at compile-time

This commit introduces the numeric compile-time constants

- MBEDTLS_SSL_CONF_MIN_MINOR_VER
- MBEDTLS_SSL_CONF_MAX_MINOR_VER
- MBEDTLS_SSL_CONF_MIN_MAJOR_VER
- MBEDTLS_SSL_CONF_MAX_MAJOR_VER

which, when defined, overwrite the runtime configurable fields
mbedtls_ssl_config::min_major_ver etc. in the SSL configuration.

As for the preceding case of the ExtendedMasterSecret configuration,
it also introduces and puts to use getter functions for these variables
which evaluate to either a field access or the macro value, maintaining
readability of the code.

The runtime configuration API mbedtls_ssl_conf_{min|max}_version()
is kept for now but has no effect if MBEDTLS_SSL_CONF_XXX are set.
This is likely to be changed in a later commit but deliberately omitted
for now, in order to be able to study code-size benefits earlier in the
process.
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index 169e054..ee8bd81 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -1132,10 +1132,18 @@
     unsigned int dhm_min_bitlen;    /*!< min. bit length of the DHM prime   */
 #endif
 
+#if !defined(MBEDTLS_SSL_CONF_MAX_MAJOR_VER)
     unsigned char max_major_ver;    /*!< max. major version used            */
+#endif /* !MBEDTLS_SSL_CONF_MAX_MAJOR_VER */
+#if !defined(MBEDTLS_SSL_CONF_MAX_MINOR_VER)
     unsigned char max_minor_ver;    /*!< max. minor version used            */
+#endif /* !MBEDTLS_SSL_CONF_MAX_MINOR_VER */
+#if !defined(MBEDTLS_SSL_CONF_MIN_MAJOR_VER)
     unsigned char min_major_ver;    /*!< min. major version used            */
+#endif /* !MBEDTLS_SSL_CONF_MIN_MAJOR_VER */
+#if !defined(MBEDTLS_SSL_CONF_MIN_MINOR_VER)
     unsigned char min_minor_ver;    /*!< min. minor version used            */
+#endif /* !MBEDTLS_SSL_CONF_MIN_MINOR_VER */
 
     /*
      * Flags (bitfields)