bufmgr code cleanup (revised)

I posted some bufmgr cleanup a few weeks ago, but it conflicted with
some concurrent changes Jan was making to the bufmgr. Here's an
updated version of the patch -- it should apply cleanly to CVS
HEAD and passes the regression tests.

This patch makes the following changes:

- remove the UnlockAndReleaseBuffer() and UnlockAndWriteBuffer()
macros, and replace uses of them with calls to the appropriate
functions.

- remove a bunch of #ifdef BMTRACE code: it is ugly & broken
(i.e. it doesn't compile)

- make BufferReplace() return a bool, not an int

- cleanup some logic in bufmgr.c; should be functionality
equivalent to the previous code, just cleaner now

- remove the BM_PRIVATE flag as it is unused

- improve a few comments, etc.

-Neil