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()`.
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 */