portlist: fix "readdirent: no such file or directory" errors on Linux.

This could happen when a process disappeared while we were reading its
file descriptor list.

I was able to replicate the problem by running this in another
terminal:

    while :; do for i in $(seq 10); do
      /bin/true & done >&/dev/null; wait >&/dev/null;
    done

And then running the portlist tests thousands of times.

Fixes #339.

Signed-off-by: Avery Pennarun <apenwarr@tailscale.com>

portlist/portlist_linux.go

@@ -110,6 +110,12 @@ func addProcesses(pl []Port) ([]Port, error) {
 			if err == io.EOF {
 				return nil
 			}
+			if os.IsNotExist(err) {
+				// This can happen if the directory we're
+				// reading disappears during the run. No big
+				// deal.
+				return nil
+			}
 			if err != nil {
 				return fmt.Errorf("addProcesses.readDir: %w", err)
 			}
@@ -155,6 +161,12 @@ func foreachPID(fn func(pidStr string) error) error {
 		if err == io.EOF {
 			return nil
 		}
+		if os.IsNotExist(err) {
+			// This can happen if the directory we're
+			// reading disappears during the run. No big
+			// deal.
+			return nil
+		}
 		if err != nil {
 			return fmt.Errorf("foreachPID.readdir: %w", err)
 		}