Allow forcing nullness of columns during bootstrap.

Bootstrap determines whether a column is null based on simple builtin
rules. Those work surprisingly well, but nonetheless a few existing
columns aren't set correctly. Additionally there is at least one patch
sent to hackers where forcing the nullness of a column would be helpful.

The boostrap format has gained FORCE [NOT] NULL for this, which will be
emitted by genbki.pl when BKI_FORCE_(NOT_)?NULL is specified for a
column in a catalog header.

This patch doesn't change the marking of any existing columns.

Discussion: 20150215170014.GE15326@awork2.anarazel.de

src/backend/bootstrap/bootscanner.l

@@ -109,6 +109,9 @@ insert			{ return(INSERT_TUPLE); }
 "on"			{ return(ON); }
 "using"			{ return(USING); }
 "toast"			{ return(XTOAST); }
+"FORCE"			{ return(XFORCE); }
+"NOT"			{ return(XNOT); }
+"NULL"			{ return(XNULL); }
 
 {arrayid}		{
 					yylval.str = MapArrayTypeName(yytext);