Correctly exit from namespace on error (re: f73b8617)

The referenced commit introduced at least one bug: EXIT traps were
not triggered if a special builtin threw an error in a namespace
that is within a virtual subshell.

src/cmd/ksh93/sh/xec.c: sh_exec(): TNSPACE:
- When an error occurs, siglongjmp to the previous saved
  environment; it is not correct to sh_exit directly.

src/cmd/ksh93/tests/namespace.sh:
- Remove the forkign workaround and the TODO where I incorrectly
  blamed this problem on the virtual subshell mechanism.

src/cmd/ksh93/sh/xec.c

@@ -2523,7 +2523,7 @@ int sh_exec(register const Shnode_t *t, int flags)
 				sh_popcontext(chkp);
 				enter_namespace(oldnspace);
 				if(jmpval)	/* error occurred */
-					sh_exit(sh.exitval);
+					siglongjmp(*sh.jmplist,jmpval);
 				break;
 			}
 #endif /* SHOPT_NAMESPACE */

src/cmd/ksh93/tests/namespace.sh

@@ -133,11 +133,8 @@ set +o allexport
 # ======
 # A namespace's parent scope should be restored after an error occurs.
 # https://github.com/ksh93/ksh/issues/479#issuecomment-1140514159
-#
-# TODO: this test fails when run in a non-forking comsub: the trap
-# is not executed -- the zillionth bug with virtual subshells >:-/
 exp=$'123 456\n789 456'
-got=$(ulimit -t unlimited 2>/dev/null  # workaround: force fork
+got=$(
 	trap 'echo $x ${.test.x}; x=789; echo $x ${.test.x}' EXIT
 	x=123
 	namespace test