Proposal 171 changes broke ‘tor2web mode’

A ‘git bisect’ traced the problem to the proposal 171 changes, and then to commit e8b9815711e7cd1ef0b83153aefcc0d05e817f4e.

This commit causes circuit_has_opened to send two ESTABLISH_RENDEZVOUS cells on each rendezvous circuit, because connection_ap_attach_pending can never attach a stream to a CIRCUIT_PURPOSE_C_ESTABLISH_REND circuit. ‘wanoskarnet’ says that hidden service client support seemed to still be working in non-tor2web-mode versions of 0.2.3.x because they cannibalized pre-built three-hop circuits, and circuit_has_opened is never called for those.

This is also the cause of legacy/trac#4171 (moved) (wanoskarnet diagnosed this, too).