And '/' in the suffix DN now works.

When upgrading, may need to slapcat/slapadd the database if back-ldif
is used as a general backend with chars like ':' or Windows' special
chars in DNs.

Change dn2path(), get_entry() and move_entry() prototypes/usage.
Remove unused statements/initializations.  get_entry() now returns
a result code, reducing result code handling a bit.

So make olcDbDirectory SINGLE-VALUE and set SLAP_DBFLAG_ONE_SUFFIX.
Take advantage of that in ldif_back_referrals().

slap_send_ldap_result() when rs->sr_flags & REP_REF_MUSTBEFREED:
- Do not return with rs->sr_ref == <freed data>, set it to NULL instead.
- Plug memory leak when op->o_domain_scope || op->o_protocol<LDAP_VERSION3.
send_ldap_response():
- Paranoia: Reset REP_MATCHED_MUSTBEFREED and REP_REF_MUSTBEFREED,
  in case the caller reuses the SlapReply without resetting the flags.

regular mutex_lock.