doc: typo for Channels.pm, mention files for which we are the master.

doc: typo for Channels.pm, mention files for which we are the master.

* HACKING: mention files for which we are the master.
* lib/Automake/ChannelDefs.pm: typo for Automake::Channels;
mention that we hold the master copy of this.
* lib/Automake/Channels.pm: update/add text for master copy notice.
* lib/Automake/Configure_ac.pm: likewise.
* lib/Automake/FileUtils.pm: likewise.
* lib/Automake/Getopt.pm: likewise.
* lib/Automake/XFile.pm: likewise.