sort: replace home made line reader by getdelim(3)

The previous code had bug when reading lines with an unexpected
encoding, returning without the full line being captured.
This result in sort complaining with "sort: Illegal byte sequence"

Using getdelim(3) instead of the home made code, fixes the situation.

PR: 241679
Reported by: Ronald F. Guilmette <rfg-freebsd@tristatelogic.com>
MFC After: 1 week
Reviewed by: markj, imp
Differential Revision: https://reviews.freebsd.org/D36948