Peercoin: Fix no listen mode
          Without network discovery addresses broadcasted by peers
          are not recorded by IP address manager, causing connection
          issues for no listen mode (-listen=0).

Symptom:
    Connection issues under no listen mode (listen=0)

Diagnosis:
    vfReachable is not initialized if network discovery (Discover())
    is skipped.
    Message handler (ProcessMessage()) would then not pass the addresses
    received from peers (message 'addr') to IP address manager (addrman),
    resulting in insufficient connections under no listen mode.
    vfReachable is introduced in commit 090e5b40. So it appears to be a
    bitcoin regression introduced since bitcoin v0.7.