examples: bistromathic: demonstrate the use of yyexpected_tokens

Let's use GNU readline and its TAB autocompletion to demonstrate the
use of yyexpected_tokens.

This shows a number of weaknesses in our current approach:

- some macros (yyssp, etc.) from push parsers "leak" in user code, we
  need to undefine them

- the context needed by yyexpected_tokens does not need the token,
  yypstate actually suffices

- yypstate is not properly setup when first allocated, which results
  in a crash of yyexpected_tokens if fired before a first token was
  read.  We should move initialization from yypush_parse into
  yypstate_new.

* examples/c/bistromathic/parse.y (yylex): Take input as a string, not
a file.
(EXIT): New token.
(input): Adjust to work only on a line.
(line): Remove.
(symbol_count, process_line, expected_tokens, completion)
(init_readline): New.
* examples/c/bistromathic/bistromathic.test: Adjust expectations.

NEWS

@@ -104,9 +104,11 @@ GNU Bison NEWS
   The lexcalc example (a simple example in C based on Flex and Bison) now
   also demonstrates location tracking.
 
-  A new C example, bistromathic, is a fully featured calculator using many
-  Bison features: pure interface, location tracking, internationalized
-  custom error messages, lookahead-correction, rich debug traces, etc.
+  A new C example, bistromathic, is a fully featured interactive calculator
+  using many Bison features: pure interface, push parser, autocompletion
+  based on the current parser state (using yyexpected_tokens), location
+  tracking, internationalized custom error messages, lookahead-correction,
+  rich debug traces, etc.
 
 * Noteworthy changes in release 3.5.2 (2020-02-13) [stable]