Use umask() to set file and UNIX socket permissions without race conditions.

Use umask() to set file and UNIX socket permissions without race conditions.

As mentioned by Erik Tews, calling fchmod() after fopen() leaves a small window
for exploits. As long as tinc is single-threaded, we can use umask() instead to
reduce file permissions. This also works when creating the AF_UNIX control socket.

The umask of the user running tinc(d) is used for most files, except for the
private keys, invitation files, PID file and control socket.