Support #[unix_sigpipe = "inherit|sig_dfl|sig_ign"] on fn main()

This makes it possible to instruct libstd to never touch the signal
handler for `SIGPIPE`, which makes programs pipeable by default (e.g.
with `./your-program | head -n 1`) without `ErrorKind::BrokenPipe`
errors.

src/test/ui/attributes/unix_sigpipe/unix_sigpipe-only-feature.rs

@@ -0,0 +1,13 @@
+// run-pass
+// aux-build:sigpipe-utils.rs
+
+#![feature(unix_sigpipe)]
+
+fn main() {
+    extern crate sigpipe_utils;
+
+    // Only #![feature(unix_sigpipe)] is enabled, not #[unix_sigpipe = "..."].
+    // This shall not change any behavior, so we still expect SIGPIPE to be
+    // ignored
+    sigpipe_utils::assert_sigpipe_handler(sigpipe_utils::SignalHandler::Ignore);
+}