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".
Per GDB the "Start of New Year Procedure", this patch
- renames the current ChangeLog into ChangeLog-2015;
- starts a new ChangeLog file.
gdb/ChangeLog:
* config/djgpp/fnchange.lst: Add entry for gdb/ChangeLog-2015.