at daemon 3.1.20, with 3 fixes

From the Debian BUG system I incorporated 3 fixes. One of them is experimental. It fixes a broken code but may have side effects. Please test it.

• New release 3.1.20:

   * Add option b to getopt, (Closes: #812972).
   * Comment a possible broken code, (Closes: #818508).
   * Add a fflush to catch more errors during writes, (Closes: #801186).

You may download from here at_3.1.20.orig.tar.gz.