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>

src/cmd/ksh93/include/version.h

@@ -21,7 +21,7 @@
 
 #define SH_RELEASE_FORK	"93u+m"		/* only change if you develop a new ksh93 fork */
 #define SH_RELEASE_SVER	"1.0.0-beta.2"	/* semantic version number: https://semver.org */
-#define SH_RELEASE_DATE	"2022-02-01"	/* must be in this format for $((.sh.version)) */
+#define SH_RELEASE_DATE	"2022-02-02"	/* must be in this format for $((.sh.version)) */
 #define SH_RELEASE_CPYR	"(c) 2020-2022 Contributors to ksh " SH_RELEASE_FORK
 
 /* Scripts sometimes field-split ${.sh.version}, so don't change amount of whitespace. */