Allow xtt_save_to_file to set file permissions

[drbild]

xtt_save_to_file is used by enftun to save the certificate and private key. The private key should not be world-readable, which is the default when writing a file (on many systems).

In util/file_io.h/c:

• Add the file permissions mask as a third argument to xtt_save_to_file
• Add wrapper function xtt_save_key_to_file with mask 0600
• Add wrapper function xtt_save_cert_to_file with mask 0644.