Fix BUG_CASEEMPT: empty 'case' list was syntax error

'case x in esac' should be syntactically correct, but was an error:

	$ ksh -c 'case x in esac'
	ksh: syntax error at line 1: `case' unmatched

Inserting a newline was a workaround:

	$ ksh -c $'case x in\nesac'
	(no output)

The problem was that the 'esac' reserved word was not being
recognised if it immediately followed the 'in' reserved word.

src/cmd/ksh93/sh/lex.c: sh_lex():
- Do not turn off recognition of reserved words after 'in' if we're
  in a 'case' construct; only do this for 'for' and 'select'.

src/cmd/ksh93/tests/case.sh:
- Add seven regression test for correct recognition of 'esac'.
  Only two failed on ksh93. The rest is to catch future bugs.

Fixes: https://github.com/ksh93/ksh/issues/177

src/cmd/ksh93/sh/lex.c

@@ -1503,7 +1503,7 @@ breakloop:
 			}
 			else
 				lp->lex.skipword = 0;
-			if(c==INSYM)
+			if(c==INSYM && !lp->lex.incase)
 				lp->lex.reservok = 0;
 			else if(c==TIMESYM)
 			{