man: update example with kimchi directives

The example was using tlstunnel directives.
contrib/systemd: add template service file
Fix path for site configs with subtree roots

This makes site configs such as the following work:

    site foo.example.org/foo/ {
        file_server /path/to/index/html/
man: better describe path matching rules
makefile: remove build dep from install target

Otherwise the Go tooling gets run as root.
Set the X-Forwarded-* headers
man: document Forwarded for reverse_proxy
Set the Forwarded header
Read remote addr from proxy header

This allows X-Forwarded-For headers to have the correct value.
Remove unused Listener.Server
build: make kimchi target PHONY
man: document redirect directive
Add redirect directive
Improve config parsing error messages
Disable directory listings by default

The file_server.browse directive allows to explicitly enable them.

Closes: https://todo.sr.ht/~emersion/kimchi/3
Rename root directive to file_server
Extract backend handling
Add import directive
Add basic_auth directive

Closes: https://todo.sr.ht/~emersion/kimchi/6
Add a header directive

Closes: https://todo.sr.ht/~emersion/kimchi/7