scd: Fix a race condition for new_reader_slot.

* scd/apdu.c (reader_table_lock, apdu_init): New.
(new_reader_slot): Serialize by reader_table_lock.
* scd/app.c (lock_app, unlock_app, app_new_register): Fix error code
usage.
(initialize_module_command): Call apdu_init.
* scd/scdaemon.c (main): Handle error for initialize_module_command.

This is a long standing bug. There are two different things; The
serialization of allocating a new SLOT, and the serialization of using
the SLOT. The latter was implemented in new_reader_slot by lock_slot.
However, the former was not done. Thus, there was a possible race where
a same SLOT is allocated to multiple threads.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>