doc: standardize messages about the '-' stdin FILE

doc: standardize messages about the '-' stdin FILE

* src/system.h (emit_stdin_note): A new function, refactoring
the usage note about the '-' FILE implying stdin.
* src/base64.c (usage): Use the new function to emit the
note in a standard location and with standard separation.
* src/cat.c (usage): Likewise.
* src/csplit.c (usage): Likewise.
* src/cut.c (usage): Likewise.
* src/expand.c (usage): Likewise.
* src/fmt.c (usage): Likewise.
* src/head.c (usage): Likewise.
* src/md5sum.c (usage): Likewise.
* src/nl.c (usage): Likewise.
* src/od.c (usage): Likewise.
* src/paste.c (usage): Likewise.
* src/pr.c (usage): Likewise.
* src/ptx.c (usage): Likewise.
* src/shred.c (usage): Likewise.
* src/shuf.c (usage): Likewise.
* src/sort.c (usage): Likewise.
* src/sum.c (usage): Likewise.
* src/tac.c (usage): Likewise.
* src/tail.c (usage): Likewise.
* src/tsort.c (usage): Likewise.
* src/unexpand.c (usage): Likewise.
* src/wc.c (usage): Likewise.
* src/join.c (usage): Adjust the separation used for
the message referring to FILE1 or FILE2 as stdin.
* src/comm.c (usage): Add a message using the same
wording (translation) as used in join.
* src/split.c (usage): Reword to using FILE rather than
INPUT, allowing use of emit_stdin_note().  Also remove
the mention of "fixed-size" pieces as this isn't now
always the case.
Fixes http://pad.lv/1450179