util-lib: optionally, when writing a string to a file, verify string on failure

util-lib: optionally, when writing a string to a file, verify string on failure

With this change, the idiom:

    r = write_string_file(p, buf, 0);
    if (r < 0) {
           if (verify_one_line_file(p, buf) > 0)
                   r = 0;
    }

gets reduced to:

    r = write_string_file(p, buf, WRITE_STRING_FILE_VERIFY_ON_FAILURE);

i.e. when writing the string fails and the new flag
WRITE_STRING_FILE_VERIFY_ON_FAILURE is specified we'll not return a
failure immediately, but check the contents of the file. If it matches
what we wanted to write we suppress the error and exit cleanly.