Don't import/export readonly attribute via magic A__z env var

While automagically importing/exporting ksh variable attributes via
the environment is probably a misfeature in general (now disabled
for POSIX standard mode), doing so with the readonly attribute is
particularly problematic. Scripts can take into account the
possibility of importing unwanted attributes by unsetting or
typesetting variables before using them. But there is no way for a
script to get rid of an unwanted imported readonly variable. This
is a possible attack vector with no possible mitigation.

This commit blocks both the import and the export of the readonly
attribute through the environment. I consider it a security fix.

src/cmd/ksh93/sh/init.c: env_import_attributes():
- Clear NV_RDONLY from imported attributes before applying them.

src/cmd/ksh93/sh/name.c: sh_envgen():
- Remove NV_RDONLY from bitmask defining attributes to export.

NEWS

@@ -12,6 +12,9 @@ Any uppercase BUG_* names are modernish shell bug IDs.
   3. The -c/--call, -n/--name and -s/--standard options matched all variable
      names provided by 'getconf -a', even if none were actual matches.
 
+- The readonly attribute of ksh variables is no longer imported from
+  or exported to other ksh shell instances through the environment.
+
 2021-04-16:
 
 - Fixed a bug in emacs mode: after using tab completion to complete the name