The help text should probably explain how keygrip is used if it is given.

I would have expected something like

while (*line && spacep (line))
  *line++ = 0;

to skip all whitespace instead of just a single whitespace character, but I see that
other code also doesn't skip multiple whitespace characters.

This fails if the keygrip is followed by whitespace. It's unlikely, but parsing chvnostr is much more robust because it explicitly looks for 0x00 or whitespace instead of just looking for 0x00 as strlen does.