unexpand: should allow multiple files

[cakebaker]

GNU unexpand allows multiple files:

$ echo "aaa" > a
$ echo "bbb" > b
$ unexpand a b
aaa
bbb

uutils unexpand, on the other hand, only supports a single file:

$ echo "aaa" > a
$ echo "bbb" > b
$ cargo run unexpand a b
aaa

This issue was mentioned in #5845 (comment) and I think they are already working on a fix.