Properly initialize the random number generator in pw_getpass().

Right now, the automatically generated passwords have been rather
predictable. :-(

MFC after:	1 day

usr.sbin/pw/pw_user.c

@@ -1058,6 +1058,7 @@ pw_getrand(u_char *buf, int len)
 {
 	int i;
 
+	srandomdev();
 	for (i = 0; i < len; i++) {
 		unsigned long val = random();
 		/* Use all bits in the random value */