maint: use consistent references to standard files in messages

maint: use consistent references to standard files in messages

* cfg.mk (sc_standard_outputs): Add a grep command for source files.
* src/du.c (main): Use standard input instead of stdin, standard output
instead of stdout, and standard error instead of stderr in messages.
* src/nohup.c (main): Likewise.
* src/sort.c (main): Likewise.
* src/split.c (main): Likewise.
* src/stdbuf.c (main): Likewise.
* src/wc.c (main): Likewise.
* tests/du/files0-from.pl (@Tests): Adjust test case to new messages.
* tests/sort/sort-files0-from.pl: Likewise.
* tests/wc/wc-files0-from.pl: Likewise.