Backport 'printf -v' from ksh 93v-

'printf' on bash and zsh has a popular -v option that allows
assigning formatted output directly to variables without using a
command substitution. This is much faster and avoids snags with
stripping final linefeeds. AT&T had replicated this feature in the
abandoned 93v- beta version. This backports it with a few tweaks
and one user-visible improvement.

The 93v- version prohibited specifying a variable name with an
array subscript, such as printf -v var\[3\] foo. This works fine on
bash and zsh, so I see no reason why this should not work on ksh,
as nv_putval() deals with array subscripts just fine.

src/cmd/ksh93/bltins/print.c: b_print():
- While processing the -v option when called as printf, get a
  pointer to the variable, creating it if necessary. Pass only the
  NV_VARNAME flag to enforce a valid variable name, and not (as
  93v- does) the NV_NOARRAY flag to prohibit array subscripts.
- If a variable was given, set the output file to an internal
  string buffer and jump straight to processing the format.
- After processing the format, assign the contents to the string
  buffer to the variable.

src/cmd/ksh93/data/builtins.c:
- Document the new option, adding a warning that unquoted square
  brackets may trigger pathname expansion.

src/cmd/ksh93/data/builtins.c

@@ -1193,7 +1193,7 @@ const char sh_optprint[] =
 ;
 
 const char sh_optprintf[] =
-"[-1c?\n@(#)$Id: printf (ksh 93u+m) 2021-09-13 $\n]"
+"[-1c?\n@(#)$Id: printf (ksh 93u+m) 2021-11-18 $\n]"
 "[--catalog?" SH_DICT "]"
 "[+NAME?printf - write formatted output]"
 "[+DESCRIPTION?\bprintf\b writes each \astring\a operand to "
@@ -1358,6 +1358,9 @@ const char sh_optprintf[] =
 	"time conversions will be treated as if \bnow\b were supplied.]"
 "[+?\bprintf\b is equivalent to \bprint -f\b which allows additional "
 	"options to be specified.]"
+"[v]:[name?Put the output in the variable \aname\a instead of writing to "
+	"standard output. \aname\a may include an array subscript (note that "
+	"the square brackets should be quoted to avoid pathname expansion).]"
 "\n"
 "\nformat [string ...]\n"
 "\n"