Move bin/neubot to UNIX/bin/neubot

Now bin/neubot is the script to run neubot under UNIX, therefore
move it below the UNIX directory to avoid surprising people who
browses the tree.
UNIX/bin/neubot: adapt NEUBOT_HOME to new directory layout

With plugins we install $(DATADIR)/neubot/neubot (which contains
the core) and $(DATADIR)/neubot/mod_* (a directory for each plugin).

Therefore update NEUBOT_HOME to be $(DATADIR)/neubot.