dnsdist: Add an option to write grepq's output to a file

[rgacogne]

Short description

We regularly ask for the output of grepq("") while investigating issues, and it sometimes is too big to be practical to retrieve via the console.

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[omoerbeek]

Few nits