Added alternated Diffie-Hellman module.

1. Add modified files dhm.c and dhm.h

library/dhm.c

@@ -57,6 +57,9 @@
 #define mbedtls_free       free
 #endif
 
+#if !defined(MBEDTLS_DHM_ALT)
+
+
 /* Implementation that should never be optimized out by the compiler */
 static void mbedtls_zeroize( void *v, size_t n ) {
     volatile unsigned char *p = v; while( n-- ) *p++ = 0;
@@ -577,7 +580,7 @@ int mbedtls_dhm_parse_dhmfile( mbedtls_dhm_context *dhm, const char *path )
 }
 #endif /* MBEDTLS_FS_IO */
 #endif /* MBEDTLS_ASN1_PARSE_C */
-
+#endif/*MBEDTLS_DHM_ALT*/
 #if defined(MBEDTLS_SELF_TEST)
 
 static const char mbedtls_test_dhm_params[] =