Properly fix $LINENO crash on ARM (re: 23b7a163) and other bugs

The typecast fix was insufficient, avoiding the crash only when
compiling with optimisation disabled. The real problem is that
put_lineno() was passed a misaligned pointer, and that the value
didn't actually contain a double but a string. The bug occurred
when restoring the LINENO value upon exiting a virtual subshell.

Thanks to Harald van Dijk for figuring out the fix.

src/cmd/ksh93/sh/subshell.c: nv_restore():
- When restoring a special variable as defined by nv_cover(),
  do not pass either the np->nvflag bits or NV_NOFREE. Why?
  * The np->nvflag bits are not needed. They are also harmful
    because they may include the NV_INTEGER bit. This is set
    when the value is numeric. However, nv_getval() always
    returns the value in string form, converting it if it is
    numeric. So the NV_INTEGER flag should never be passed
    to nv_putval() when it uses the result of nv_getval().
  * According to nval.3, the NV_NOFREE flag stops nv_putval() from
    creating a copy of the value. But this should be unnecessary
    because the earlier _nv_unset(mp,NV_RDONLY|NV_CLONE) should
    ensure there is no previous value. In addition, the NV_NOFREE
    flag triggered another bug that caused the value of SECONDS to
    be corrupted upon restoring it when exiting a virtual subshell.
- When restoring a regular variable, copy the entire nvalue union
  and not just the 'cp' member. In practice this worked because
  no current member of the nvalue union is larger than a pointer.
  However, there is no guarantee it will stay that way.

src/cmd/ksh93/tests/leaks.sh:
- Add disabled test for a memory leak that was discovered in the
  course of dealing with this bug. The fix doesn't introduce or
  influence it. It will have to be dealt with later.

src/cmd/ksh93/tests/locale.sh:
- Add test for restoring locale on leaving virtual subshell.
  https://github.com/ksh93/ksh/issues/253#issuecomment-815290154

src/cmd/ksh93/tests/variables.sh:
- Test against corruption of SECONDS on leaving virtual subshell.
  https://github.com/ksh93/ksh/issues/253#issuecomment-815191052

Co-authored-by: Harald van Dijk <harald@gigawatt.nl>
Progresses: https://github.com/ksh93/ksh/issues/253

src/cmd/ksh93/sh/subshell.c

@@ -392,9 +392,9 @@ static void nv_restore(struct subshell *sp)
 		}
 		mp->nvflag = np->nvflag|(flags&NV_MINIMAL);
 		if(nv_cover(mp))
-			nv_putval(mp, nv_getval(np),np->nvflag|NV_NOFREE|NV_RDONLY);
+			nv_putval(mp,nv_getval(np),NV_RDONLY);
 		else
-			mp->nvalue.cp = np->nvalue.cp;
+			mp->nvalue = np->nvalue;
 		if(nofree && np->nvfun && !np->nvfun->nofree)
 			free((char*)np->nvfun);
 		np->nvfun = 0;