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/test/benchmark.c b/programs/test/benchmark.c
index 2b86566..a8f7f6e 100644
--- a/programs/test/benchmark.c
+++ b/programs/test/benchmark.c
@@ -43,7 +43,7 @@
 int main( void )
 {
     mbedtls_printf("MBEDTLS_TIMING_C not defined.\n");
-    return( 0 );
+    mbedtls_exit( 0 );
 }
 #else
 
@@ -999,7 +999,7 @@
     fflush( stdout ); getchar();
 #endif
 
-    return( 0 );
+    mbedtls_exit( 0 );
 }
 
 #endif /* MBEDTLS_TIMING_C */