Support output without delimiter, call write in loop

[andrewthad]

The recv subcommand has a mistake that can cause it to truncate messages when outputting them. It ignores error codes and doesn't call write in a loop to write out the entirety of the buffer. This PR fixes that mistake. Additionally, it adds support for a "no delimiter" option, selected with the character x.

I've not actually tested this yet, so don't merge this. I'm just planning on using this in a one-off script where the output is piped to curl, and I noticed that the output would occasionally be wrong.

[andrewthad]

I've confirmed that these changes work.

[goeb]

Could you please give more context and details to clarify why the write() function was interrupted before writing all bytes? (does the mq process receive signals, ...)
Do you have a minimal and reproducible example?

[andrewthad]

It doesn't receive any signals. I just thought it would be good for it to be correct in that case, but I can back that change out if you prefer. Or I could just change it to fail loudly if it doesn't write all the bytes on the first attempt.

[goeb]

I agree with the no-delimiter option ('x'). Why didn't you apply it to the cmd_recv_follow() function ?

But I do not agree with how you handle a hypothetical partial write. As we do not know what could cause a partial write, it is hard to handle it and avoid an infinite loop or a long blocking. I prefer the program to fail with an error message.

For the naming of your variables, please use lower_case_with_underscore.