cmd/gc: preserve safe annotation of package def.

A package file may begin as either "package foo" or
"package foo safe". The latter is relevant when using -u.
https://golang.org/cl/6903059 resulted in the distinction
being dropped when a package was read for the second or later time.
This CL records whether that "safe" tag was present,
and includes it in the dummy statement generated for the lexer.

R=golang-dev, r, minux.ma, daniel.morsing, iant
CC=golang-dev
https://golang.org/cl/8255044

src/cmd/gc/go.h

@@ -394,6 +394,7 @@ struct	Pkg
 	uchar	imported;	// export data of this package was parsed
 	char	exported;	// import line written in export data
 	char	direct;	// imported directly
+	char	safe;	// whether the package is marked as safe
 };
 
 typedef	struct	Iter	Iter;

src/cmd/gc/go.y

@@ -251,7 +251,8 @@ import_package:
 		} else if(strcmp(importpkg->name, $2->name) != 0)
 			yyerror("conflicting names %s and %s for package \"%Z\"", importpkg->name, $2->name, importpkg->path);
 		importpkg->direct = 1;
-		
+		importpkg->safe = curio.importsafe;
+
 		if(safemode && !curio.importsafe)
 			yyerror("cannot import unsafe package \"%Z\"", importpkg->path);
 	}

src/cmd/gc/lex.c

@@ -602,7 +602,7 @@ void
 importfile(Val *f, int line)
 {
 	Biobuf *imp;
-	char *file, *p, *q;
+	char *file, *p, *q, *tag;
 	int32 c;
 	int len;
 	Strlit *path;
@@ -610,8 +610,6 @@ importfile(Val *f, int line)
 
 	USED(line);
 
-	// TODO(rsc): don't bother reloading imports more than once?
-
 	if(f->ctype != CTSTR) {
 		yyerror("import statement not a string");
 		fakeimport();
@@ -686,7 +684,11 @@ importfile(Val *f, int line)
 	// to the lexer to avoid parsing export data twice.
 	if(importpkg->imported) {
 		file = strdup(namebuf);
-		p = smprint("package %s\n$$\n", importpkg->name);
+		tag = "";
+		if(importpkg->safe) {
+			tag = "safe";
+		}
+		p = smprint("package %s %s\n$$\n", importpkg->name, tag);
 		cannedimports(file, p);
 		return;
 	}