Allow std{in,out,err} to be a socket

runtime/bin/file_linux.cc

Index: runtime/bin/file_linux.cc
diff --git a/runtime/bin/file_linux.cc b/runtime/bin/file_linux.cc
index 825bdea47fabc5f68c68e0db6cfec9dbefcec901..409f31c8800b4f5e0eee3d3c790a71d101729d9e 100644
--- a/runtime/bin/file_linux.cc
+++ b/runtime/bin/file_linux.cc
@@ -205,6 +205,7 @@ File::StdioHandleType File::GetStdioHandleType(int fd) {
   }
   if (S_ISCHR(buf.st_mode)) return kTerminal;
   if (S_ISFIFO(buf.st_mode)) return kPipe;
+  if (S_ISSOCK(buf.st_mode)) return kSocket;
   if (S_ISREG(buf.st_mode)) return kFile;
   return kOther;
 }