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

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.