Replace EFF_RDONLY by EFF_PRIVATE

Replace EFF_RDONLY by EFF_PRIVATE

Read-only was a bit of a misnomer: you could write to the table by
obtaining a pointer into it from ef_ptr(), you just couldn't write to
the backing file.

Semantic changes:

* ef_flush() is now allowed when the table is file-backed or privately
  mapped.  Before, it had to be file-backed.  Flushing a privately
  mapped table does nothing, just like flushing a read-only table did.

* ef_write() is now allowed when the table is file-backed or fully
  cached.  Before, it had to be file-backed and not read-only.
  Writing to a privately mapped file-backed table doesn't write to the
  file.

* ef_extend() is not implemented for privately mapped tables, just
  like it wasn't implemented for read-only tables.