Remove the dlopen() pointer from struct filedef.

Remove the dlopen() pointer from struct filedef.

This pointer is almost never needed, and it increases the size of the filedef
struct for all files (of which there are a huge number for large builds).
Instead keep a bit field marking whether the file is a loaded object and if so
call a new function to unload it.  In load.c we keep a simple linked list of
loaded objects (of which there will be very few typically) and their dlopen()
pointers.