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.

compiler/rustc_builtin_macros/src/test_harness.rs

@@ -266,7 +266,7 @@ fn generate_test_harness(
 ///
 /// By default this expands to
 ///
-/// ```ignore UNSOLVED (I think I still need guidance for this one. Is it correct? Do we try to make it run? How do we nicely fill it out?)
+/// ```ignore (messes with test internals)
 /// #[rustc_main]
 /// pub fn main() {
 ///     extern crate test;