Support posix_spawn() for FreeBSD.

spawn() is expected to return an error if the specified file could not be
executed.  FreeBSD's posix_spawn() supports returning ENOENT/ENOEXEC if
the exec() fails, which not all platforms support.  This brings a very
significant performance improvement for FreeBSD, involving heavy use of
Command in threads, due to fork() invoking jemalloc fork handlers and
causing lock contention.  FreeBSD's posix_spawn() avoids this problem
due to using vfork() internally.

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

@@ -235,14 +235,14 @@ impl Command {
  io::Error::last_os_error()
  }
 
- #[cfg(not(any(target_os = "linux", target_os = "macos")))]
+ #[cfg(not(any(target_os = "freebsd")))]
  fn posix_spawn(&mut self, _stdio: &ChildPipes, _envp: Option<&CStringArray>)
  -> io::Result<Option<Process>>
  {
  Ok(None)
  }
 
- #[cfg(any(target_os = "linux", target_os = "macos"))]
+ #[cfg(any(target_os = "freebsd"))]
  fn posix_spawn(&mut self, stdio: &ChildPipes, envp: Option<&CStringArray>)
  -> io::Result<Option<Process>>
  {