maint: avoid '%s' quoting notation in diagnostic messages

Add a new rule to ensure the use of quote() instead of '%s' or `%s'
in format strings of diagnostics messages.

* cfg.mk (sc_prohibit_quotes_notation): Add rule.
* TODO: Remove the entry regarding the '%s' notation.
* src/mkfifo.c (main): Remove the offending and in this case even
duplicate quoting in the format string of the error diagnostic.
* src/mknod.c (main): Likewise.
* src/df.c (decode_output_arg): Change two invocations of error()
according to the above new rule.
* src/numfmt.c: Fix numerous wrong quote notations to fit the above
new rule, mostly in internal debugging diagnostic messages.

1 files changed, 8 insertions, 0 deletions

diff --git a/cfg.mk b/cfg.mk
index 788c35134..9e8e8ac4c 100644
--- a/cfg.mk
+++ b/cfg.mk
@@ -151,6 +151,14 @@ sc_system_h_headers: .re-list
 		  1>&2;  exit 1; } || :;				\
 	fi
 
+# Files in src/ should not use '%s' notation in format strings,
+# i.e., single quotes around %s (or similar) should be avoided.
+sc_prohibit_quotes_notation:
+	@cd $(srcdir)/src && GIT_PAGER= git grep -n "\".*[\`']%s'.*\"" *.c \
+	  && { echo '$(ME): '"Use quote() to avoid quoted '%s' notation" 1>&2; \
+	       exit 1; }  \
+	  || :
+
 sc_sun_os_names:
 	@grep -nEi \
 	    'solaris[^[:alnum:]]*2\.(7|8|9|[1-9][0-9])|sunos[^[:alnum:]][6-9]' \