psyc: ipc messages

git-svn-id: https://gnunet.org/svn/gnunet@31790 140774ce-b5e7-0310-ab8b-a85725594a96
env: added head() and shift()

git-svn-id: https://gnunet.org/svn/gnunet@31791 140774ce-b5e7-0310-ab8b-a85725594a96
psyc: ipc messages, notify callback for modifiers, tests

git-svn-id: https://gnunet.org/svn/gnunet@31792 140774ce-b5e7-0310-ab8b-a85725594a96
psyc: message handler checks

git-svn-id: https://gnunet.org/svn/gnunet@31793 140774ce-b5e7-0310-ab8b-a85725594a96