Fix octal number arguments in printf integer arithmetic

Bug 1: POSIX requires numbers used as arguments for all the %d,
%u... in printf to be interpreted as in the C language, so
	printf '%d\n' 010
should output 8 when the posix option is on. However, it outputs 10.

This bug was introduced as a side effect of a change introduced in
the 2012-02-07 version of ksh 93u+m, which caused the recognition
of leading-zero numbers as octal in arithmetic expressions to be
disabled outside ((...)) and $((...)). However, POSIX requires
leading-zero octal numbers to be recognised for printf, too.

The change in question introduced a sh.arith flag that is set while
we're processing a POSIX arithmetic expression, i.e., one that
recognises leading-zero octal numbers.
Bug 2: Said flag is not reset in a command substitution used within
an arithmetic expression. A command substitution should be a
completely new context, so the following should both output 10:

$ ksh -c 'integer x; x=010; echo $x'
10            # ok; it's outside ((…)) so octals are not recognised
$ ksh -c 'echo $(( $(integer x; x=010; echo $x) ))'
8             # bad; $(comsub) should create new non-((…)) context

src/cmd/ksh93/bltins/print.c: extend():
- For the u, d, i, o, x, and X conversion modifiers, set the POSIX
  arithmetic context flag before calling sh_strnum() to convert the
  argument. This fixes bug 1.

src/cmd/ksh93/sh/subshell.c: sh_subshell():
- When invoking a command substitution, save and unset the POSIX
  arithmetic context flag. Restore it at the end. This fixes bug 2.

Reported-by: @stephane-chazelas
Resolves: https://github.com/ksh93/ksh/issues/326

src/cmd/ksh93/COMPATIBILITY

@@ -151,6 +151,11 @@ For more details, see the NEWS file and for complete details, see the git log.
 	correctly negates another '!', e.g., [[ ! ! 1 -eq 1 ]] now returns
 	0/true. Note that this has always been the case for 'test'/'['.
 
+28.	In the 'printf' builtin (and the 'print -f' equivalent), numeric
+	arguments with a leading zero are now once again recognized as octal
+	numbers as in ksh93 versions before 2012-02-07, and as POSIX requires.
+	For example, 'printf "%d\n" 010' now once again outputs '8'.
+
 ____________________________________________________________________________
 
 		KSH-93 VS. KSH-88