Unify the example programs' termination

This is done to account for platforms, for which we want custom behavior
upon the program termination, hence we call `mbedtls_exit()` instead of
returning from `main()`.

For the sake of consistency, introduces the modifications have been made
to the test and utility examples as well. These, while less likely to be
used in the low level environments, won't suffer from such a change.
diff --git a/programs/random/gen_random_havege.c b/programs/random/gen_random_havege.c
index 5ea52ae..6a65de0 100644
--- a/programs/random/gen_random_havege.c
+++ b/programs/random/gen_random_havege.c
@@ -48,7 +48,7 @@
 int main( void )
 {
     mbedtls_printf("MBEDTLS_HAVEGE_C not defined.\n");
-    return( 0 );
+    mbedtls_exit( 0 );
 }
 #else
 
@@ -65,13 +65,13 @@
     if( argc < 2 )
     {
         mbedtls_fprintf( stderr, "usage: %s <output filename>\n", argv[0] );
-        return( exit_code );
+        mbedtls_exit( exit_code );
     }
 
     if( ( f = fopen( argv[1], "wb+" ) ) == NULL )
     {
         mbedtls_printf( "failed to open '%s' for writing.\n", argv[1] );
-        return( exit_code );
+        mbedtls_exit( exit_code );
     }
 
     mbedtls_havege_init( &hs );
@@ -104,6 +104,6 @@
 exit:
     mbedtls_havege_free( &hs );
     fclose( f );
-    return( exit_code );
+    mbedtls_exit( exit_code );
 }
 #endif /* MBEDTLS_HAVEGE_C */