Re: New shared memory hooks proposal (was Re: pre_load_libraries)

The attached patch provides add-ins with the means to register for
shared memory and LWLocks.  This greatly improves the ease with which
shared memory may be used from add-ins, while keeping the accounting and
management for that shared memory separate.

Specifically it adds named add-in shared memory contexts.  From these,
memory can be allocated without affecting the memory available in other
contexts.

Usage is as follows:
from add-in functions called from preload_libraries, you may call
  RegisterAddinContext(const * name, size_t size)
to register a new (logical) shared memory segment.

and
  RegisterAddinLWLock(LWLockid *lock_ptr);
to request that a LWLock be allocated, placed into *lock_ptr.

The actual creation of the shared memory segment and lwlocks is
performed later as part of shared memory initialisation.

To allocate shared memory from a named context you would use
   ShmemAllocFromContext(size_t size, const char *name);

To reset a shared memory context back to its original unused state (from
which new allocations may be performed), you may use
  ShmemResetContext(const char *name);

This works for me (for Veil) and make check runs fine.

I have not included any documentation updates in the patch as I'm not
sure where such API changes should be documented.

All comments, questions and suggestions are welcomed.

__
Marc