Fix BUG_LOOPRET2 and related return/exit misbehaviour

The 'exit' and 'return' commands without an argument failed to pass
down the exit status of the last-run command when incorporated in a
block with redirection, &&/|| list, 'case' statement, or 'while',
'until' or 'for' loop.

src/cmd/ksh93/bltins/cflow.c:
- Use $?, which is sh.savexit a.k.a. shp->savexit, as the default
  exit status value if there is no argument, instead of
  shp->oldexit. This fixes the default exit status behaviour to
  match POSIX and other shells.

src/cmd/ksh93/include/defs.h,
src/cmd/ksh93/include/shell.h:
- Remove now-unused sh.oldexit (a.k.a. shp->oldexit) private struct
  member. It appeared to fulfill the same function as sh.savexit,
  but in a slightly broken way.
- Move the savexit/$? declaration from the _SH_PRIVATE part of the
  struct definition to the public API part. Since $? uses this,
  it's clearly a publicly exposed value already, and this is
  generally the one to use. (If anything, it's exitval that should
  have been private.) This declares savexit right next to exitval,
  rewriting the comments to clarify the difference between them.

src/cmd/ksh93/sh/fault.c,
src/cmd/ksh93/sh/subshell.c,
src/cmd/ksh93/sh/xec.c:
- Remove assignments to shp->oldexit.

src/cmd/ksh93/tests/basic.sh:
- Add thorough regression tests for the default exit status
  behaviour of 'return' and 'exit' in various lexical contexts.
- Verify that 'for' and 'case' without any command, as well as a
  lone redirection, still correctly reset the exit status to 0.

Fixes: #117

src/cmd/ksh93/sh/fault.c

@@ -614,7 +614,6 @@ void sh_done(void *ptr, register int sig)
 	if(t=shp->st.trapcom[0])
 	{
 		shp->st.trapcom[0]=0; /*should free but not long */
-		shp->oldexit = savxit;
 		sh_trap(t,0);
 		savxit = shp->exitval;
 	}