Make error checking for thread functions stricter.

Make error checking for thread functions stricter.

* src/systhread.c (sys_thread_create): Change return type to bool.
Check for errors returned by pthread_attr_setstacksize and
pthread_attr_destroy.
(sys_mutex_init): Abort on errors.  Enable mutex checks when checking
is enabled.
(sys_cond_init): Abort on errors.
(sys_mutex_lock, sys_mutex_unlock, sys_cond_wait)
(sys_cond_signal, sys_cond_broadcast, sys_cond_destroy): Check for
errors in debug mode.