sort: unify the code to read from FILE *

Previously the code to read from a local file or stdin was sperarated
After the change to remove the home made line reader used for stdin
(replaced by getdelim) it apprears that the rest of the code which is
used to read from any FILE * but stdin can benefit from the exact same
change.

(cherry picked from commit 8b9071360a312f2bb2104864af0b9905acac87bc)