bufmgr code cleanup

This patch cleans up some of the bufmgr code:

- replace uses of

LockBuffer(buf, BUFFER_LOCK_UNLOCK);
ReleaseBuffer(buf);

with the equivalent, but more concise:

UnlockAndReleaseBuffer(buf);

- analogous changes were made by replacing LockBuffer() + WriteBuffer()
with UnlockAndWriteBuffer()

- remove a bunch of #ifdef BMTRACE code, since it was ugly and broken
anyway

- remove an unused buffer descriptor bit flag (BM_PRIVATE)

- move the definition of INVALID_DESCRIPTOR to out of bufmgr.h and into
freelist.c, since it is the only file that uses it

- remove another unused function, and fix a few comments

Please apply to the CVS HEAD.

-Neil