std: Refactor process spawning on Unix

* Build up the argp/envp pointers while the `Command` is being constructed
  rather than only when `spawn` is called. This will allow better sharing of
  code between fork/exec paths.
* Rename `child_after_fork` to `exec` and have it only perform the exec half of
  the spawning. This also means the return type has changed to `io::Error`
  rather than `!` to represent errors that happen.

src/libstd/process.rs

@@ -209,7 +209,9 @@ impl Command {
     /// Add multiple arguments to pass to the program.
     #[stable(feature = "process", since = "1.0.0")]
     pub fn args<S: AsRef<OsStr>>(&mut self, args: &[S]) -> &mut Command {
-        self.inner.args(args.iter().map(AsRef::as_ref));
+        for arg in args {
+            self.arg(arg.as_ref());
+        }
         self
     }
 

src/libstd/sys/unix/ext/process.rs

@@ -12,8 +12,8 @@
 
 #![stable(feature = "rust1", since = "1.0.0")]
 
-use os::unix::raw::{uid_t, gid_t};
 use os::unix::io::{FromRawFd, RawFd, AsRawFd, IntoRawFd};
+use os::unix::raw::{uid_t, gid_t};
 use process;
 use sys;
 use sys_common::{AsInnerMut, AsInner, FromInner, IntoInner};