Improve --strip-trailing-cr handling:

• Advance ctold for f1 and ctnew for f2
• ungetc() if the character is unexpected
• Don't break early when we hit the combination on one side

PR: 230049
Reported by: maskray <emacsray gmail com>
Reviewed by: bapt, maskray
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D16451