This reduces the implicit requirement from 5.6.2 back to 5.6,
while raising the explicit requirement to match the actual code.

* configure.ac (PERL): Fail up front if perl is too old.
* NEWS: Document this.
* README: Likewise.
* README-hacking: Likewise.
* lib/Autom4te/ChannelDefs.pm: Bump requirement.
* lib/Autom4te/General.pm: Relax requirement.