seq: fix check for rounding error/truncation

Based on OpenBSD 30f0fd29ba6c:

We need to compare the printable version of the last value displayed,
not the floating point representation. Otherwise, we may print the
last value twice.

PR: 271964
Reported by: Daniel Kolesa
Reviewed by: yuripv
Obtained from: OpenBSD
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40601

(cherry picked from commit e54db9a9ccd588f650a2e57bf9d1cbbafc0e12eb)