doc: clarify --zero-terminated option

doc: clarify --zero-terminated option

* src/join.c (usage): Reword to avoid implication that
the NUL byte is only generated as the output delimeter.
* src/sort.c (usage): Likewise.
* src/shuf.c (usage): Likewise. Also since we're changing the
translation string take the opportunity to separate out
the description to a separate string to reduce translation overhead.
* src/uniq.c (usage): Likewise.
* src/stty.c (usage): s/null/NUL/ for consistency.
* src/basename.c (usage): Reword for accuracy/consistency.
* src/dirname.c (usage): Likewise.
* src/du.c (usage): Likewise.
* src/env.c (usage): Likewise.
* src/printenv.c (usage): Likewise.
* src/readlink.c (usage): Likewise.
* src/realpath.c (usage): Likewise.
* doc/coreutils.texi: Consolidate/share the descriptions of
--null, --zero and --zero-terminated.