bootstrap: pure parser and reentrant scanner

Use the flex %option reentrant and the bison option %pure-parser to
make the generated scanner and parser pure, reentrant, and
thread-safe.

Make the generated scanner use palloc() etc. instead of malloc() etc.

For the bootstrap scanner and parser, reentrancy and memory management
aren't that important, but we make this change here anyway so that all
the scanners and parsers in the backend use a similar set of options
and APIs.

Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Reviewed-by: Andreas Karlsson <andreas@proxel.se>
Discussion: https://www.postgresql.org/message-id/flat/eb6faeac-2a8a-4b69-9189-c33c520e5b7b@eisentraut.org

src/backend/bootstrap/bootparse.y

@@ -34,10 +34,6 @@
 
 #include "bootparse.h"
 
-/* silence -Wmissing-variable-declarations */
-extern int boot_yychar;
-extern int boot_yynerrs;
-
 
 /*
  * Bison doesn't allocate anything that needs to live across parser calls,
@@ -81,6 +77,9 @@ static int num_columns_read = 0;
 
 %}
 
+%parse-param {yyscan_t yyscanner}
+%lex-param   {yyscan_t yyscanner}
+%pure-parser
 %expect 0
 %name-prefix="boot_yy"
 
@@ -141,6 +140,8 @@ Boot_OpenStmt:
 					do_start();
 					boot_openrel($2);
 					do_end();
+
+					(void) yynerrs; /* suppress compiler warning */
 				}
 		;