purple: fix memory leak in set buddy status

Reported in this forum thread

  https://sourceforge.net/p/sipe/discussion/688534/thread/4aca1c0a/
transport: fix potential memory leak when signing message

Another leak reported in this forum thread

  https://sourceforge.net/p/sipe/discussion/688534/thread/4aca1c0a/

NOTE: I could not reproduce it so this fix is just a guess.
Fix #262: Adium: SIPE doesn't auto- or re-connect

If server auto-discovery is in use then the plugin doesn't have a value
for KEY_CONNECT_HOST. We used to set it to the localized name returned
by NSHost. But that isn't necessarily a valid DNS hostname and therefore
Adium's reachability test fails for the SIPE account.

Now we hard-code the value to "localhost" instead. During my tests this
was the only value that always worked.
Fix #262: Adium: SIPE doesn't auto- or re-connec (II)

If server auto-discovery is in use then use the well known host
"adium.im" instead of "localhost". This prevents the SIPE account from
going online when there is no network.

NOTE: this will mean that SIPE will not auto-connect for users that are
      on an Intranet-only setup, because they will not be able to reach
      the host adium.im on the Internet.
srtp: parse encryption key from SDP
srtp: generate encryption key for each stream
srtp: offer encryption key when initiating a call
srtp: implement sipe_backend_media_set_encryption_keys()
notify: store ucPC2PCAVEncryption value form server
srtp: add sipe_backend_media_get_encryption_policy()
media: send encryption SDP attribute based on policy
media: add stream_decryption_keys list to sipe_media_private
media: refactor encryption compatibility check
srtp: negotiate encryption and apply keys to backend
srtp: encryption policies WIP
media: mux RTP and RTCP when using TCP connection

Lync uses a single TCP connection for both components.
media: duplicate TCP candidates
filetransfer: use 'init' callback in sipe_file_transfer

Replaces sipe_core_ft_incoming_init() and sipe_core_ft_outgoing_init()
and the specific callback method is now set by Sipe core depending on
whether incoming or outgoing file transfer is being handled.

Will allow for some code to be shared between legacy OCS file transfer
implementation and the new Lync approach.
filetransfer: use 'cancelled' callback in sipe_file_transfer

Replaces sipe_core_ft_cancel().
filetransfer: use 'deallocate' callback in sipe_file_transfer

Replaces sipe_core_ft_deallocate().