Convert unquote_*_word users to expect isempty(p) after the last entry

Convert unquote_*_word users to expect isempty(p) after the last entry

This is so that, when called in a loop, unquote_first_word can
distinguish between reaching the end of a string because it has consumed
all the input before the end, and consuming all the input.

This is important because we later add a flag that allows

    char *in = "";
    char *out;
    unquote_first_word(&in, &out, flags);

To put "" in out, and set in = NULL, so the trailing empty string of the
input can be consumed, and mark that the input has been consumed.