Show the HTTP request headers and the client IP on the /status page.
This is for debugging request headers added by reverse proxies such as
Cloudflare and Nginx, and for making sure the client IP is correctly set
by the X-Forwarded-For header.
The `source` command is a bash-ism and doesn't work in a strictly POSIX
shell like dash, which is the /bin/sh on Debian/Ubuntu. Use `.` instead.
https://en.wikipedia.org/wiki/Dot_(command)
