Move declarations of different message types to message-base.h.
Move send functions to message-send.c/h.
Move receive functions to message-receive.c/h.

Created notifyfs-fsevent.c/h, to describe the event.

These changes we're necessary to split the different areas:

- handling of data shared in shared memory between local processes, like the entry, inode and attr.

- sending of messages.

- receiving of messages.

- describing a fsevent.