awk: allow printf('%c') to output NUL, closes 13486

Treat the output of printf as binary rather than a null-terminated
string so that NUL characters can be output.

This is considered to be a GNU extension, though it's also available
in mawk and FreeBSD's awk.

function                                             old     new   delta
evaluate                                            3487    3504     +17
awk_printf                                           504     519     +15
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 2/0 up/down: 32/0)               Total: 32 bytes

Signed-off-by: Ron Yorston <rmy@pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

testsuite/awk.tests

@@ -383,6 +383,11 @@ testing "awk errors on missing delete arg" \
 	"awk -e '{delete}' 2>&1" "awk: cmd. line:1: Too few arguments\n" "" ""
 SKIP=
 
+optional FEATURE_AWK_GNU_EXTENSIONS
+testing "awk printf('%c') can output NUL" \
+	"awk '{printf(\"hello%c null\n\", 0)}'" "hello\0 null\n" "" "\n"
+SKIP=
+
 # testing "description" "command" "result" "infile" "stdin"
 testing 'awk negative field access' \
 	'awk 2>&1 -- '\''{ $(-1) }'\' \