I know this is a throwaway joke, but I was interested anyway...

According to my own local Markdown formatters, the answer would be both "usr" and "bin", with the surrounding slashes removed, but the internal slash remaining. In other words:

usr/bin

(but underlined instead of italic!)

Of course, this problem is nothing new since a filename might easily be named `_my_file_`.

You put in in ~code~ or =verbatim= markup, thusly: =/usr/bin/=

None because it would be surrounded by backticks.

But also, this convention predated rendering text/plain as text/html

Same as in my_snake_case_name :)