SHA2 renamed to SHA256, SHA4 renamed to SHA512 and functions accordingly

The SHA4 name was not clear with regards to the new SHA-3 standard. So
SHA2 and SHA4 have been renamed to better represent what they are:
SHA256 and SHA512 modules.
diff --git a/programs/test/benchmark.c b/programs/test/benchmark.c
index 7ee465f..acccd01 100644
--- a/programs/test/benchmark.c
+++ b/programs/test/benchmark.c
@@ -181,33 +181,33 @@
                     ( hardclock() - tsc ) / ( j * BUFSIZE ) );
 #endif
 
-#if defined(POLARSSL_SHA2_C)
+#if defined(POLARSSL_SHA256_C)
     printf( HEADER_FORMAT, "SHA-256" );
     fflush( stdout );
 
     set_alarm( 1 );
     for( i = 1; ! alarmed; i++ )
-        sha2( buf, BUFSIZE, tmp, 0 );
+        sha256( buf, BUFSIZE, tmp, 0 );
 
     tsc = hardclock();
     for( j = 0; j < 1024; j++ )
-        sha2( buf, BUFSIZE, tmp, 0 );
+        sha256( buf, BUFSIZE, tmp, 0 );
 
     printf( "%9lu Kb/s,  %9lu cycles/byte\n", i * BUFSIZE / 1024,
                     ( hardclock() - tsc ) / ( j * BUFSIZE ) );
 #endif
 
-#if defined(POLARSSL_SHA4_C)
+#if defined(POLARSSL_SHA512_C)
     printf( HEADER_FORMAT, "SHA-512" );
     fflush( stdout );
 
     set_alarm( 1 );
     for( i = 1; ! alarmed; i++ )
-        sha4( buf, BUFSIZE, tmp, 0 );
+        sha512( buf, BUFSIZE, tmp, 0 );
 
     tsc = hardclock();
     for( j = 0; j < 1024; j++ )
-        sha4( buf, BUFSIZE, tmp, 0 );
+        sha512( buf, BUFSIZE, tmp, 0 );
 
     printf( "%9lu Kb/s,  %9lu cycles/byte\n", i * BUFSIZE / 1024,
                     ( hardclock() - tsc ) / ( j * BUFSIZE ) );