Export all variables assigned to while allexport is on (#431)

All variables that are assigned a value should be exported while
the allexport shell option is enabled. This works in most cases,
but variables assigned to with ${var:=foo} or $((var=123)) aren't
exported while allexport is on.

src/cmd/ksh93/sh/name.c:
- nv_putval(): This is the central assignment function; all forms
  of variable assignment end up here. So this is the best place
  to check for SH_ALLEXPORT and turn on the export attribute.
- nv_setlist(): Remove allexport handling, now redundant.

src/cmd/ksh93/bltins/read.c: sh_readline():
- Remove allexport handling, now redundant.

src/cmd/ksh93/sh/main.c: sh_main():
- nv_putval() is used to initialize PS4 and IFS using nv_putval();
  this is after an -a/--allexport specified on the ksh command
  line has been processed, so temporarily turn that off.

Co-authored-by: Martijn Dekker <martijn@inlv.org>

NEWS

@@ -3,6 +3,12 @@ For full details, see the git log at: https://github.com/ksh93/ksh/tree/1.0
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2022-02-02:
+
+- Fixed the -a/allexport option to export all variables that are assigned
+  values. Previously, arithmetic $((var=1)) or conditional ${var:=value}
+  assignments were not exported even with allexport on.
+
 2022-02-01:
 
 - Upon invocation, the interactive shell no longer leaves the user without