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/pkey/key_app_writer.c b/programs/pkey/key_app_writer.c
index 6096429..b0b831a 100644
--- a/programs/pkey/key_app_writer.c
+++ b/programs/pkey/key_app_writer.c
@@ -94,7 +94,7 @@
 int main( void )
 {
     mbedtls_printf( "MBEDTLS_PK_PARSE_C and/or MBEDTLS_PK_WRITE_C and/or MBEDTLS_FS_IO not defined.\n" );
-    return( 0 );
+    mbedtls_exit( 0 );
 }
 #else
 
@@ -436,6 +436,6 @@
     fflush( stdout ); getchar();
 #endif
 
-    return( exit_code );
+    mbedtls_exit( exit_code );
 }
 #endif /* MBEDTLS_PK_PARSE_C && MBEDTLS_PK_WRITE_C && MBEDTLS_FS_IO */