eina_binbuf: allow expand & usage of extra bytes.

eina_binbuf: allow expand & usage of extra bytes.

Some code needs to read directly into eina_binbuf to avoid an extra
copy from eina_binbuf_append* variants.

This can be achieved by using eina_binbuf_expand(), which returns a
write-able slice of the spare bytes. Once they are used,
eina_binbuf_use() should be called to increment "buf->len", which is
used by all other binbuf functions such as eina_binbuf_length_get() or
eina_binbuf_append_slice().