compile-time format checking

We've had a few pull requests fixing format/argument mismatches
lately.  I did't notice when PRINTF_F(format_index,first_arg_index)
attribute use and the checking gcc and clang do with it got removed,
but that was very useful.  Putting it back triggers a whole bunch
of "format string is not literal" warnings, but that's because
'-Wformat-nonliteral' was explicitly added to the *.2020 hints.
Checking pline/You/&c arguments in the cases where the format is a
literal is more valuable than the complaints for sprintf being fed
a generated format, so reinstate PRINTF_F usage and turn off the
check for non-literal format strings.

sys/unix/hints/include/compiler.2020

@@ -1,5 +1,5 @@
 #------------------------------------------------------------------------------
-# NetHack 3.7  compiler.2020 $NHDT-Date: 1597332785 2020/08/13 15:33:05 $  $NHDT-Branch: NetHack-3.7 $
+# NetHack 3.7  compiler.2020 $NHDT-Date: 1637992237 2021/11/27 05:50:37 $  $NHDT-Branch: NetHack-3.7 $
 
 # compiler flags:  CCFLAGS is used to construct a value for CFLAGS with
 #	various -I, -D, and -W settings appended below;
@@ -47,7 +47,7 @@ CFLAGS+=-Wall -Wextra -Wno-missing-field-initializers \
 	-Wreturn-type -Wunused -Wformat -Wswitch -Wshadow -Wwrite-strings
 CFLAGS+=-pedantic
 CFLAGS+=-Wmissing-declarations
-CFLAGS+=-Wformat-nonliteral
+CFLAGS+=-Wno-format-nonliteral
 CFLAGS+=-Wunreachable-code
 #
 # the following are not allowed in C++
@@ -63,7 +63,7 @@ CCXXFLAGS+=-Wall -Wextra -Wno-missing-field-initializers \
         -Wreturn-type -Wunused -Wformat -Wswitch -Wshadow -Wwrite-strings
 CCXXFLAGS+=-pedantic
 CCXXFLAGS+=-Wmissing-declarations
-CCXXFLAGS+=-Wformat-nonliteral
+CCXXFLAGS+=-Wno-format-nonliteral
 
 ifeq "$(CCISCLANG)" ""
 # gcc-specific follows