Mass .gitignore update for .orig.c files

Due to the fact we now have .orig.c files and thus the possibility of
winner.orig or prog.orig being built I have updated all the .gitignore
files so that these files ignored. This was done with the following
shell commands, for reference:

    for i in $(find 19* 200* 201[0123] -mindepth 2 -maxdepth 2 -name .gitignore) ; do echo $(dirname "$i"|sed -e 's,[0-9]\{4\}/,,g').orig >> "$i"; done
    for i in $(find 201[4-9] 2020 -mindepth 2 -maxdepth 2 -name .gitignore) ; do echo prog.orig >> "$i"; done

and the two commands is because it was only in 2014 that the winning
entries were renamed to prog so prog.orig is the correct glob.

1990/theorem/.gitignore

@@ -6,3 +6,5 @@ sorter
 sorter.c
 theorem_bkp
 theorem_bkp.c
+theorem.orig
+prog.orig