bulk change, see long comment for details

* seq internal type definitions (seqPvt.h):
  - grouped struct members according to quasi-static vs. dynamic
  - factored out struct assigned_channel from struct channel
  - removed struct queue_entry, use new seq_queue module instead
  - added stuff needed for safe mode to struct state_set_control_block:
    USER_VAR pointer, an array of dirty flags (one per channel)
* refactored and simplified thread initialization and shutdown,
  removed the death1/2/3/4 cruft
* removed auxiliary thread (and related types); Jeff Hill confirmed
  that this is redundant if CA is used with preemptive callbacks enabled
* removed all remaining global variables, except in seqCommands where
  they get initialized before multithreading starts; pvSys now gets
  initialized inside seqAddProg (with a global mutex taken)
* conceptually separate program (which gets registered before
  multithreading starts) and running instance, add instance number
  to the latter to distinguish them (in addition to thread id)
* completed implementation of safe mode:
  - additions to pvPut, pvGet, pvGetQ for anonymous pvs
  - anonymous pvs now work exactly as named ones
* re-worked SYNC/ASYNC pvPut/pvGet, so that they work as documented
* adapted documentation