-o posix: inverse-sync braceexpand; properly sync letoctal

{Brace,expansion} is potentially incompatible with POSIX scripts,
because in POSIX those are simple literal strings with no special
meaning. So the POSIX option should really turn that off.

As of b301d417, the 'posix' option was also forcing 'letoctal'
behaviour on, without actually setting that option. I've since
found that to be a botch; 'let' may recognise octals without that
option being set, and that looks like a bug.

So as of this commit, the '-o posix' option actually toggles both
of these options off/on and on/of, respectively. 'set +o posix'
toggles them inversely. However, it is now possible to control both
options (and their associated behaviour) independently in between
'set -o posix' and 'set +o posix'. Much better.

src/cmd/ksh93/sh/main.c: sh_main():
- If SH_POSIX was set on init, turn on SH_LETOCTAL by default
  instead of SH_BRACEEXPAND.

src/cmd/ksh93/sh/args.c: sh_applyopts():
- Turn off SH_BRACEEXPAND and turn on SH_LETOCTAL when SH_POSIX is
  turned on (but not if it was already on).
- Turn on SH_BRACEEXPAND and turn off SH_LETOCTAL when SH_POSIX is
  turned off (but not if it was already off).

src/cmd/ksh93/sh/arith.c: arith():
- Revert to pre-b301d417 and only check SH_LETOCTAL option when
  deciding whether 'let' should skip initial zeros.

src/cmd/ksh93/tests/options.sh:
- Update $- test to allow '-o posix' to switch B = braceexpand.

src/cmd/ksh93/sh.1:
- Update.
- Edit for clarity.

NEWS

@@ -3,6 +3,14 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2020-09-18:
+
+- Setting the 'posix' option now turns off the 'braceexpand' option, as brace
+  expansion is not specified by POSIX and potentially incompatible with sh
+  scripts. In addition, 'set -o posix' now turns on the 'letoctal' option
+  instead of controlling that behaviour directly. 'set +o posix' does the
+  reverse of these.
+
 2020-09-17:
 
 - In the vi and emacs line editors, repeat count parameters can now also be