Properly terminate strings received from the daemon

Hi,

  the data received using net_receive() is raw, not a zero-terminated C string. It must be terminated, otherwise the aps topic sent to the registering client is broken and maybe other bad stuff could happen.

This PR also contains some other small changes:
 * Fix compilation with dovecot 2.2 (network.h vs. net.h).
 * Enable the plugin for LMTP as well in case it is used instead of LDA

Please consider for inclusion.

Cheers,
  -Mike