Fix shell exit on function call redirection error (re: 23f2e23)

This regression also exists on ksh 93v- and ksh2020, from which it
was backported.

Reproducer:

$ (fn() { true; }; fn >/dev/null/ne; true) 2>/dev/null; echo $?
1

Expected output: 0 (as on ksh 93u+).

FreeBSD sh and NetBSD sh are the only other known shells that share
this behaviour. POSIX currently allows both behaviours, but may
require the ksh 93u+ behaviour in future. In any case, this causes
an incompatibility with established ksh behaviour that could easily
break existing ksh scripts.

src/cmd/ksh93/sh/xec.c: sh_exec():
- Commit 23f2e23 introduced a check for jmpval > SH_JMPIO (5).
  When a function call pushes context for a redirection, this is
  done with the jmpval exit value of SH_JMPCMD (6). Change that to
  SH_JMPIO to avoid triggering that check.

src/cmd/ksh93/tests/exit.sh:
- Add regression tests for exit behaviour on various kinds of
  shell errors as listed in the POSIX standard, including an error
  in a redirection of a function call.

Fixes: https://github.com/ksh93/ksh/issues/310

src/cmd/ksh93/sh/xec.c

@@ -1550,7 +1550,7 @@ int sh_exec(register const Shnode_t *t, int flags)
 					if(io)
 					{
 						indx = shp->topfd;
-						sh_pushcontext(shp,buffp,SH_JMPCMD);
+						sh_pushcontext(shp,buffp,SH_JMPIO);
 						jmpval = sigsetjmp(buffp->buff,0);
 					}
 					if(jmpval == 0)