Rollup merge of #124584 - Nilstrieb:entrypointy, r=fee1-dead

Various improvements to entrypoint code

This moves some code around and adds some documentation comments to make it easier to understand what's going on with the entrypoint logic, which is a bit complicated.

The only change in behavior is consolidating the error messages for unix_sigpipe to make the code slightly simpler.

Author: matthiaskrgr