Update atomic writes logic to create hidden files in same directory as the files being updated

[dhurley]

Proposed changes

• If updating a file called /etc/nginx/nginx.conf, agent will first create a backup called /etc/nginx/.nginx.conf.agent.backup.
• Then it will download the file and write the updates to another hidden file called /etc/nginx/.nginx.conf.agent.tmp.
• It will then rename /etc/nginx/.nginx.conf.agent.tmp to /etc/nginx/nginx.conf.
• If the changes need to be rolled back, agent will rename /etc/nginx/.nginx.conf.agent.backup to /etc/nginx/nginx.conf.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• I have run make install-tools and have attached any dependency changes to this pull request
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• If applicable, I have updated any relevant documentation (README.md)
• If applicable, I have tested my cross-platform changes on Ubuntu 22, Redhat 8, SUSE 15 and FreeBSD 13