gc: correct buffer size in sprint, add -L to show full paths

R=ken2
CC=golang-dev
https://golang.org/cl/2338041

src/cmd/gc/doc.go

@@ -32,6 +32,8 @@ Flags:
 		output file, default 6.out for 6g, etc.
 	-e
 		normally the compiler quits after 10 errors; -e prints all errors
+	-L
+		show entire file path when printing line numbers in errors
 	-I dir1 -I dir2
 		add dir1 and dir2 to the list of paths to check for imported packages
 	-N

src/cmd/gc/lex.c

@@ -436,7 +436,9 @@ importfile(Val *f, int line)
 	path = f->u.sval;
 	if(islocalname(path)) {
 		cleanbuf = mal(strlen(pathname) + strlen(path->s) + 2);
-		sprint(cleanbuf, "%s/%s", pathname, path->s);
+		strcpy(cleanbuf, pathname);
+		strcat(cleanbuf, "/");
+		strcat(cleanbuf, path->s);
 		cleanname(cleanbuf);
 		path = strlit(cleanbuf);
 	}

src/cmd/gc/subr.c

@@ -934,6 +934,8 @@ Lconv(Fmt *fp)
 				break;
 			fmtprint(fp, " ");
 		}
+		if(debug['L'])
+			fmtprint(fp, "%s/", pathname);
 		if(a[i].line)
 			fmtprint(fp, "%s:%ld[%s:%ld]",
 				a[i].line->name, lno-a[i].ldel+1,