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-start.rs

@@ -0,0 +1,6 @@
+#![feature(start)]
+#![feature(unix_sigpipe)]
+
+#[start]
+#[unix_sigpipe = "inherit"] //~ error: `unix_sigpipe` attribute can only be used on `fn main()`
+fn custom_start(argc: isize, argv: *const *const u8) -> isize { 0 }