It's not necessary to explicitly initialize these to false

might as well make this a bool while here

also no need for whitespace between the global variables

generally these should be in alpha order

perhaps just stdin_files()

This is a tricky case; I'd prefer not to break a string in the middle (so that e.g. grep "cannot be less than zero" works to find the source of a message).

It looks a bit awkward but

errx(1,
    "%s: ...",
    optarg);

is probably best