eina: add Eina_File API.

eina: add Eina_File API.

NOTE: the purpose of this API is to replace mmap user in the
EFL, share cache and more code across them. The potential user
are eet, evas, efreet, eio and enlil. More patch are needed for
them to use this infra. Help welcome :-)

NOTE2: this API also need more test and is waiting for some
more pthread infra before being thread safe. But at the end
it will be thread safe if eina thread safety is requested.

git-svn-id: http://svn.enlightenment.org/svn/e/trunk/eina@58637 7cbeb6ba-43b4-40fd-8cce-4c39aea84d33