Rename identifiers containing double-underscore

Rename identifiers containing double-underscore (`__`) to avoid `__`.
The reason to avoid double-underscore is that all identifiers
containing double-underscore are reserved in C++. Rename all such
identifiers that appear in any public header, including ssl_internal.h
which is in principle private but in practice is installed with the
public headers.

This commit makes check-names.sh pass.

```
perl -i -pe 's/\bMBEDTLS_SSL__ECP_RESTARTABLE\b/MBEDTLS_SSL_ECP_RESTARTABLE_ENABLED/g; s/\bMBEDTLS_KEY_EXCHANGE_(_\w+)_(_\w+)\b/MBEDTLS_KEY_EXCHANGE${1}${2}/g' include/mbedtls/*.h library/*.c programs/*/*.c scripts/data_files/rename-1.3-2.0.txt tests/suites/*.function
```
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index 1666f8c..e93e8f3 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -825,7 +825,7 @@
 typedef void mbedtls_ssl_async_cancel_t( mbedtls_ssl_context *ssl );
 #endif /* MBEDTLS_SSL_ASYNC_PRIVATE */
 
-#if defined(MBEDTLS_KEY_EXCHANGE__WITH_CERT__ENABLED) &&        \
+#if defined(MBEDTLS_KEY_EXCHANGE_WITH_CERT_ENABLED) &&        \
     !defined(MBEDTLS_SSL_KEEP_PEER_CERTIFICATE)
 #define MBEDTLS_SSL_PEER_CERT_DIGEST_MAX_LEN  48
 #if defined(MBEDTLS_SHA256_C)
@@ -841,7 +841,7 @@
 /* This is already checked in check_config.h, but be sure. */
 #error "Bad configuration - need SHA-1, SHA-256 or SHA-512 enabled to compute digest of peer CRT."
 #endif
-#endif /* MBEDTLS_KEY_EXCHANGE__WITH_CERT__ENABLED &&
+#endif /* MBEDTLS_KEY_EXCHANGE_WITH_CERT_ENABLED &&
           !MBEDTLS_SSL_KEEP_PEER_CERTIFICATE */
 
 /*
@@ -937,7 +937,7 @@
     void *p_vrfy;                   /*!< context for X.509 verify calllback */
 #endif
 
-#if defined(MBEDTLS_KEY_EXCHANGE__SOME__PSK_ENABLED)
+#if defined(MBEDTLS_KEY_EXCHANGE_SOME_PSK_ENABLED)
     /** Callback to retrieve PSK key from identity                          */
     int (*f_psk)(void *, mbedtls_ssl_context *, const unsigned char *, size_t);
     void *p_psk;                    /*!< context for PSK callback           */
@@ -1000,7 +1000,7 @@
     void *p_async_config_data; /*!< Configuration data set by mbedtls_ssl_conf_async_private_cb(). */
 #endif /* MBEDTLS_SSL_ASYNC_PRIVATE */
 
-#if defined(MBEDTLS_KEY_EXCHANGE__WITH_CERT__ENABLED)
+#if defined(MBEDTLS_KEY_EXCHANGE_WITH_CERT_ENABLED)
     const int *sig_hashes;          /*!< allowed signature hashes           */
 #endif
 
@@ -1013,7 +1013,7 @@
     mbedtls_mpi dhm_G;              /*!< generator for DHM                  */
 #endif
 
-#if defined(MBEDTLS_KEY_EXCHANGE__SOME__PSK_ENABLED)
+#if defined(MBEDTLS_KEY_EXCHANGE_SOME_PSK_ENABLED)
 
 #if defined(MBEDTLS_USE_PSA_CRYPTO)
     psa_key_handle_t psk_opaque; /*!< PSA key slot holding opaque PSK.
@@ -1044,7 +1044,7 @@
                                      *   Its value is non-zero if and only if
                                      *   \c psk is not \c NULL or \c psk_opaque
                                      *   is not \c 0. */
-#endif /* MBEDTLS_KEY_EXCHANGE__SOME__PSK_ENABLED */
+#endif /* MBEDTLS_KEY_EXCHANGE_SOME_PSK_ENABLED */
 
 #if defined(MBEDTLS_SSL_ALPN)
     const char **alpn_list;         /*!< ordered list of protocols          */
@@ -2668,7 +2668,7 @@
                               mbedtls_pk_context *pk_key );
 #endif /* MBEDTLS_X509_CRT_PARSE_C */
 
-#if defined(MBEDTLS_KEY_EXCHANGE__SOME__PSK_ENABLED)
+#if defined(MBEDTLS_KEY_EXCHANGE_SOME_PSK_ENABLED)
 /**
  * \brief          Configure a pre-shared key (PSK) and identity
  *                 to be used in PSK-based ciphersuites.
@@ -2815,7 +2815,7 @@
                      int (*f_psk)(void *, mbedtls_ssl_context *, const unsigned char *,
                                   size_t),
                      void *p_psk );
-#endif /* MBEDTLS_KEY_EXCHANGE__SOME__PSK_ENABLED */
+#endif /* MBEDTLS_KEY_EXCHANGE_SOME_PSK_ENABLED */
 
 #if defined(MBEDTLS_DHM_C) && defined(MBEDTLS_SSL_SRV_C)
 
@@ -2920,7 +2920,7 @@
                               const mbedtls_ecp_group_id *curves );
 #endif /* MBEDTLS_ECP_C */
 
-#if defined(MBEDTLS_KEY_EXCHANGE__WITH_CERT__ENABLED)
+#if defined(MBEDTLS_KEY_EXCHANGE_WITH_CERT_ENABLED)
 /**
  * \brief          Set the allowed hashes for signatures during the handshake.
  *                 (Default: all available hashes except MD5.)
@@ -2941,7 +2941,7 @@
  */
 void mbedtls_ssl_conf_sig_hashes( mbedtls_ssl_config *conf,
                                   const int *hashes );
-#endif /* MBEDTLS_KEY_EXCHANGE__WITH_CERT__ENABLED */
+#endif /* MBEDTLS_KEY_EXCHANGE_WITH_CERT_ENABLED */
 
 #if defined(MBEDTLS_X509_CRT_PARSE_C)
 /**