maint: use clearerr on stdin when appropriate

maint: use clearerr on stdin when appropriate

This is so that commands like ‘fmt - -’ read from stdin
both times, even when it is a tty.  Fix some other minor
issues that are related.
* src/blake2/b2sum.c (main):
* src/cksum.c (cksum):
* src/cut.c (cut_file):
* src/expand-common.c (next_file):
* src/fmt.c (fmt):
* src/fold.c (fold_file):
* src/md5sum.c (digest_file, digest_check):
* src/nl.c (nl_file):
* src/od.c (check_and_close):
* src/paste.c (paste_parallel, paste_serial):
* src/pr.c (close_file):
* src/sum.c (bsd_sum_file):
Use clearerr on stdin so that stdin can be read multiple times
even if it is a tty.  Do not assume that ferror preserves errno as
POSIX does not guarantee this.  Coalesce duplicate diagnostic
calls.
* src/blake2/b2sum.c (main):
* src/fmt.c (main, fmt):
Report read error, even if it's merely fclose failure.
* src/fmt.c: Include die.h.
(fmt): New arg FILE.  Close input (reporting error) if not stdin.
All callers changed.
* src/ptx.c (swallow_file_in_memory): Clear stdin's EOF flag.
* src/sort.c (xfclose): Remove unnecessary feof call.