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.