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.
Clean up the SIGINT handler.
Really retry outgoing connections immediately if requested.

The retry() function would only abort connections that were in progress of
being made, it wouldn't reschedule the outgoing connections that had been
sleeping.
Non-zero exit code when reloading config file fails after SIGHUP.

When reloading the configuration file via the tinc command, the user will get
an error message if reloading has failed. However, no such warning exists when
sending a HUP signal. Previously, tincd would exit in both cases, but with a
zero exit code. Now it will exit with code 1 when reloading fails after a
SIGHUP, but tincd will keep running if it is signaled via the tinc command.
Instead, the tinc command will exit with a non-zero exit code.
Fix a typo.