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Re: (ITS#6104) race condition with cancel operation

Well, this turned out a lot of problems.  Currently blocked by 
ITS#6138 Bad Cancel/Abandon/"internal abandon"/Syncprov interactions.

back-relay/op.c done (ITS#6133). syncprov/retcode issues may fit better
under ITS#6138.

Bitflags should be unnecessary as such, just need a set of values.
Though it wouldn't hurt if they were chosen so one could use bit
operations to reduce checks for "o_abandon == A or B".  "Suppress
response" value mentioned in ITS#6138 also needed, maybe that must be a
bitflag when combined with Cancel stuff.

Some tidbits uglifying a fix:
* op->o_conn->c_mutex is locked when be->be_abandon() is called,
  but not when be->be_cancel() is called.
* send_ldap_ber() needs the lock while it holds &conn->c_write1_mutex,
  but code elsewhere indicates the reverse lock order.

  syncprov_op_abandon() holds &si->si_ops_mutex when setting
  so->s_op->o_abandon.  Does it need that?  The function needs to
  grab op->o_conn->c_mutex when called as Cancel, but must not do
  that while holding &si->si_ops_mutex since Abandon grabs the
  locks in the opposite order.