Stack robustness fixes from OpenSUSE

Three OpenSUSE patches from:
https://build.opensuse.org/package/show/shells/ksh

As usual, the relevant bug is not currently public:
https://bugzilla.opensuse.org/show_bug.cgi?id=844071

src/cmd/ksh93/sh/xec.c: sh_debug()/sh_exec():
- Fix stk restoration. [bnc#844071]

src/lib/libast/misc/stk.c:
- Fix stk aliasing code. [bnc#844071]
  (ksh93-stkalias.dif)
- Make a unknown location fatal in stkset() so that we get a core
  dump right away instead of later in an unrelated part of code.
  (ksh93-stkset-abort.dif)

src/lib/libast/man/stk.3,
src/lib/libast/man/stak.3:
- Update manual with new stkset() behaviour. (93u+m addition)
  (Note that stak is implemented as macros that translate to stk)

src/lib/libast/man/stak.3

@@ -109,8 +109,9 @@ the given \fIaddress\fP, and sets the current object to the given
 \fIaddress\fP.
 The top of the current object is set to \fIoffset\fP bytes from
 current object.
-If \fIaddress\fP is not the address of an object on the
-stack the result is undefined.
+If \fIaddress\fP is null, the stack is reset to the beginning.
+If it is non-null, but is not the address of an object on the
+stack, the program aborts and dumps core.
 .PP
 The remaining functions are used to build the current object incrementally.
 An object that is built incrementally on the stack will