* exceptions.cc (set_process_mask): Set pending signals only when signals

* exceptions.cc (set_process_mask): Set pending signals only when signals
become unmasked.
* sigproc.cc (pending_signals): Flip back to a global.
(wait_sig): Don't set pending signals when there is an armed semaphore or
signal is blocked.
* shared.cc (shared_info::initialize): Add a username parameter for user-mode
mounts.  Reorganize to try to avoid startup race.
(memory_init): Move some stuff into shared_info::initialize.
* shared_info.h (shared_info::initialize): Change declaration.
(CURR_SHARED_MAGIC): Update.