I spotted a few misplaced entries in the ChangeLog-2019 entries, and
went on to fix them.
Looking around I saw a good number of other entries in other years.
Then OCD got the best of me and I fixed them all.
Also fixes cases of wrong paths in entries, like "* gdb/foo.c" instead
of "* foo.c".
* ChangeLog-2006: New file, containing all the entries for 2006.
* ChangeLog: Removed all 2006 entries, and changed the reference
to the previous ChangeLog to point to ChangeLog 2006.
* top.c (print_gdb_version): Update copyright year.
