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

NEWS

@@ -5,6 +5,9 @@ Any uppercase BUG_* names are modernish shell bug IDs.
 
 2021-02-15:
 
+- Fixed a regression introduced by ksh93 (was not in ksh88): an empty 'case'
+  list on a single line ('case x in esac') was a syntax error.
+
 - Fixed a bug in the emacs built-in editor, introduced on 2020-09-17, that
   made the Meta-D and Meta-H keys delete single characters instead of words.