lib9: fix openbsd build

Fix build error and warning on OpenBSD 5.1,
as reported by Dimitri Sokolyuk.

R=rsc
http://codereview.appspot.com/6350044

src/lib9/_p9dialparse.c

@@ -3,6 +3,7 @@
 #include <libc.h>
 
 #include <sys/types.h>
+#include <sys/socket.h>
 #include <netdb.h>
 #include <sys/un.h>
 #include <netinet/in.h>

src/lib9/dirread.c

@@ -22,7 +22,7 @@ mygetdents(int fd, struct dirent *buf, int n)
 static int
 mygetdents(int fd, struct dirent *buf, int n)
 {
-	long off;
+	off_t off;
 	return getdirentries(fd, (void*)buf, n, &off);
 }
 #elif defined(__sun__) || defined(__NetBSD__)