caddyfile: Prevent invalid site addresses with comma

[francislavoie]

Some users forget to use a comma between their site addresses. This is invalid (commas aren't a valid character in domains) and later parts of the code like certificate automation will try to use this otherwise, which doesn't make sense. Best to error as early as possible.

Example thread on the forums where this happened: https://caddy.community/t/simplify-caddyfile/13281/9

foo.example.com,bar.example.com
respond "foo"

👇

$ caddy adapt
2021/08/22 19:29:44.950 INFO    using adjacent Caddyfile
adapt: Caddyfile:1 - Error during parsing: Site addresses cannot contain a comma ',': 'foo.example.com,bar.example.com' - put a space after the comma to separate site addresses

[mholt]

Nice, thanks. This does seem to be a common mistake.