libdevattr - Visibility change, stop memory waste

libdevattr - Visibility change, stop memory waste

* Change the visibility of some function prototypes in devattr.h to be
  only visible from within libdevattr (or when LIBDEVATTR_INTERNAL is
  specified).

* Stop wasting memory when reading the socket. Since we receive the size
  of the remaining data first, we can allocate exactly the right amount
  instead of wasting it big time by allocating 12MB no matter what.