join,sort,uniq: with -z, treat '\n' as a field separator
* NEWS: Mention the change in behavior.
* doc/coreutils.texi (newlineFieldSeparator): A new description,
referenced from ({join,sort,uniq} invocation).
* src/system.h (field_sep): A new inline function to determine
if a character is a field separator.
* src/join.c (usage): s/whitespace/blank/ to be more accurate
wrt which characters are field separators.
(xfields): s/isblank/field_sep/.
* src/sort.c (inittables): Likewise.
* src/uniq.c (find_field): Likewise.
* tests/misc/join.pl: Adjust -z test, and add a test/example
for processing the whole record with field processing.
* tests/misc/sort.pl: Add -z test cases, including case with '\n'.
* tests/misc/uniq.pl: Add -z -f test case with \n.