fix "y" to work with NUL in the RHS

* sed/execute.c (do_list) [case 'y']: Handle NUL bytes
in the RHS of a y/LHS/RHS/ transliteration.
* testsuite/y-zero.good: New test-related files.
* testsuite/y-zero.inp:
* testsuite/y-zero.sed:
* testsuite/Makefile.am (SEDTESTS): Add y-zero here.
* testsuite/Makefile.tests: And here.
* NEWS (Bug fixes): Describe it.
The bug was reported by table@inventati.org,
with the execute.c change mostly by Paolo Bonzini.
maint: correct failing "version" test

* configure.ac (COPYRIGHT_YEAR): Update to 2014, so
the "version" test passes once again.
* doc/sed.1: Also commit this auto-generated-yet-VC'd file.