Disable useless -Wformat-zero-length

It is part of -Wformat, which is enabled by -Wall.  Empty format strings are
well defined and it is perfectly reasonable to expect them in a formatting
interface.

share/mk/bsd.sys.mk

@@ -186,6 +186,9 @@ FORMAT_EXTENSIONS=	-fformat-extensions
 CWARNFLAGS+=	-Wno-unknown-pragmas
 .endif # IGNORE_PRAGMA
 
+# This warning is utter nonsense
+CWARNFLAGS+=	-Wno-format-zero-length
+
 # We need this conditional because many places that use it
 # only enable it for some files with CLFAGS.$FILE+=${CLANG_NO_IAS}.
 # unconditionally, and can't easily use the CFLAGS.clang=

sys/conf/kern.mk

@@ -61,8 +61,7 @@ CWARNEXTRA?=	-Wno-error=address				\
 CWARNEXTRA+=	-Wno-error=misleading-indentation		\
 		-Wno-error=nonnull-compare			\
 		-Wno-error=shift-overflow			\
-		-Wno-error=tautological-compare			\
-		-Wno-format-zero-length
+		-Wno-error=tautological-compare
 .endif
 .if ${COMPILER_VERSION} >= 70200
 CWARNEXTRA+=	-Wno-error=memset-elt-size
@@ -80,6 +79,9 @@ NO_WNONNULL=	-Wno-nonnull
 .endif
 .endif
 
+# This warning is utter nonsense
+CWARNFLAGS+=	-Wno-format-zero-length
+
 # External compilers may not support our format extensions.  Allow them
 # to be disabled.  WARNING: format checking is disabled in this case.
 .if ${MK_FORMAT_EXTENSIONS} == "no"