Fail if log file already exists - #74

This prevents an attack like the one described
[here](https://owasp.org/www-community/vulnerabilities/Insecure_Temporary_File#:~:text=On%20Unix%20based,with%20elevated%20permissions.).
An unprivileged attacker could use symlinks to trick a privileged
logging process to follow a symlink from the log dir and write logs over
an arbitrary file.

The components of the log names are program, host, username, tag, date,
time and PID. These are all predictable. It's not at all unusual for the
logdir to be writable by unprivileged users, and one of the fallback
directories (/tmp) traditionally has broad write privs with the sticky
bit set on Unix systems.

As a concrete example, let's say I've got a glog-enabled binary running
as a root cronjob. I can gauge when that cron job will run and then use
a bash script to spray the log dir with glog-looking symlinks to
`/etc/shadow` with predicted times and PIDs. When the cronjob runs, the
`os.Create` call will follow the symlink, truncate `/etc/shadow` and
then fill it with logs.

This change defeats that by setting `O_EXCL`, which will cause the open
call to fail if the file already exists.

Fixes CVE-2024-45339

cl/712795111 (google-internal)