Fix += operator regressions (re: fae8862c) (#270)

The bugfix for BUG_CMDSPASGN backported in commit fae8862c caused
two regressions with the += operator:

1. The += operator did not append to variables. Reproducer:
     $ integer foo=3
     $ foo+=2 command eval 'echo $foo'
     2

2. The += operator ignored the readonly attribute, modifying readonly
   variables in the same manner as above. Reproducer
     $ readonly bar=str
     $ bar+=ing command eval 'echo $bar'
     ing

Both of the regressions above were caused by nv_putval() failing to
clone the variable from the previous scope into the invocation-local
scope. As a result, 'foo+=2' was effectively 0 + 2 (since ksh didn't
clone 3). The first regression was noticed during the development of
ksh93v-, so to fix both bugs I've backported the bugfix for the
regression from the ksh93v- 2013-10-10 alpha version:
https://www.mail-archive.com/ast-users@lists.research.att.com/msg00369.html

src/cmd/ksh93/sh/name.c:
- To fix both of the bugs above, find the variable to modify with
  nv_search(), then clone it into the invocation local scope. To
  fix the readonly bug as well, this is done before the NV_RDONLY
  check (otherwise np will be missing that attribute and be
  incorrectly modified in the invocation-local scope).
- Update a nearby comment describing what sh_assignok() does (per this
  comment: https://github.com/ksh93/ksh/pull/249#issuecomment-811381759)

src/cmd/ksh93/tests/builtins.sh:
- Add regression tests for both of the now fixed regressions,
  loosely based on the regression tests in ksh93v-.

NEWS

@@ -3,6 +3,17 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2021-04-11:
+
+- Fixed two related regressions introduced on 2020-06-16:
+  1. The += assignment failed to append the value of variables when used
+     in an invocation-local scope. The following should print '5', but
+     the regression resulted in '3' being printed instead:
+       $ integer foo=2; foo+=3 command eval 'echo $foo'
+       3
+  2. Any += assignment used in an invocation-local scope could modify
+     readonly variables.
+
 2021-04-10:
 
 - Fixed: the internal count of the recursion level for arithmetic expressions