Remove the concept of thread priorities from empthread.h. Only LWP

Remove the concept of thread priorities from empthread.h.  Only LWP
supports priorities.  Update synchronization used to rely on them,
which naturally worked only with LWP (#1504036).  With that fixed, no
uses of priorities remained, but a minor bug did: players could starve
out threads with priorities below PP_PLAYER, i.e. delete_lostitems()
and player_kill_idle().  Closes #1458175:
(empth_create): Remove parameter prio.  Callers changed.  Also gets
rid of misleading comments in pthread.c and ntthread.c.
(PP_MAIN, PP_UPDATE, PP_SHUTDOWN, PP_SCHED, PP_TIMESTAMP, PP_PLAYER)
(PP_ACCEPT, PP_KILLIDLE): Remove.
(empth_init, empth_create) [EMPTH_LWP]: Pass priority 1.