Fix typos in text and comments and in http resp handling in code

[skatsubo]

• Readme
• Messages and comments in code
• requestbody = reqbody.Bytes() (duplicated in the response handling) -> responsebody = respbody.Bytes()

[alexflint]

This, actually, is intentional. You need two setsid calls to fully daemonize a process. This could use some explanation, but you can verify that

$ httptap -- setsid curl http://monasticacademy.org

actually works, which is not what we want (the purpose of this section of the readme is to give an example that doesn't work without --no-exit), whereas

$ httptap -- setsid setsid curl http://monasticacademy.org

fails, which is what we want here.

[skatsubo]

Thanks for the explanation. Fixed/reverted this line.
Yeah, I suspected some gotchas here 😄 Especially as the Python code example uses just one os.setsid().
(I was going to try it while creating the PR, but the released version from February does not have --no-exit, and when compiling from main I encountered various unrelated hiccups due to my dev environment)

[alexflint]

Hey @skatsubo, thanks for this! I have one minor comment, which is that the double-invocation of setsid was actually intentional. Other than that, thanks for submitting this!