Minor cleanups.

Use ssize_t for handling the result of a readlink call.

Fix the following compiler warning:

  system/core/toolbox/ls.c: In function 'listfile_maclabel':
  system/core/toolbox/ls.c:238:68: warning: unused parameter 'flags' [-Wunused-parameter]
   static int listfile_maclabel(const char *path, struct stat *s, int flags)

Change-Id: Idd65f3349fb9dfb6f7a374c81f72d3169af33505