Split g_file_open() into _ro() and _rw() variants

Rename g_file_open() to g_file_open_rw(), and add a new g_file_open_ro()
call that wraps the common g_file_open_ex(file, 1, 0, 0, 0) idiom. This
will make the file access mode more explicit in the code.

Change all calls to g_file_open() to the _ro() or _rw() variant as
appropriate, and replace g_file_open_ex(file, 1, 0, 0, 0) with the _ro()
call.

Lastly, add tests for the two new calls to test_os_calls.c (code
courteously provided by matt335672).

sesman/chansrv/clipboard_file.c

@@ -462,7 +462,7 @@ clipboard_send_file_data(int streamId, int lindex,
               "nPositionLow %d cbRequested %d", streamId, lindex,
               nPositionLow, cbRequested);
     g_snprintf(full_fn, 255, "%s/%s", cfi->pathname, cfi->filename);
-    fd = g_file_open_ex(full_fn, 1, 0, 0, 0);
+    fd = g_file_open_ro(full_fn);
     if (fd == -1)
     {
         LOG(LOG_LEVEL_ERROR, "clipboard_send_file_data: file open [%s] failed: %s",