maint: define usage note about mandatory args centrally

Each program with at least one long option which is marked as
'required_argument' and which has also a short option for that
option, should print a note about mandatory arguments.
Define that well-known note centrally and use it rather than
literal printf/fputs, and add it where it was missing.

* src/system.h (emit_mandatory_arg_note): Add new function.

* src/cp.c (usage): Use it rather than literal printf/fputs.
* src/csplit.c, src/cut.c, src/date.c, src/df.c, src/du.c:
* src/expand.c, src/fmt.c, src/fold.c, src/head.c, src/install.c:
* src/kill.c, src/ln.c, src/ls.c, src/mkdir.c, src/mkfifo.c:
* src/mknod.c, src/mv.c, src/nl.c, src/od.c, src/paste.c:
* src/pr.c, src/ptx.c, src/shred.c, src/shuf.c, src/sort.c:
* src/split.c, src/stdbuf.c, src/tac.c, src/tail.c, src/timeout.c:
* src/touch.c, src/truncate.c, src/unexpand.c, src/uniq.c:
Likewise.

* src/base64.c (usage): Add call of the above new function
because at least one long option has a required argument.
* src/basename.c, src/chcon.c, src/date.c, src/env.c:
* src/nice.c, src/runcon.c, src/seq.c, src/stat.c, src/stty.c:
Likewise.

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