minor smgr code cleanup

This patch cleans up some code in the smgr and elsewhere:

     - Update comment in IsReservedName() to the present day, remove
       "ugly coding for speed" as this is no longer a performance
       critical function

     - Improve some variable & function names in commands/vacuum.c. I
       was planning to rewrite this to avoid lappend(), but since I
       still intend to do the list rewrite, there's no need for that.

     - Update some smgr comments which seemed to imply that we still
       forced all dirty pages to disk at commit-time.

     - Replace some #ifdef DIAGNOSTIC code with assertions.

     - Make the distinction between OS-level file descriptors and
       virtual file descriptors a little clearer in a few comments

     - Other minor comment improvements in the smgr code

Unless anyone objects, I intend to apply this code in about 48 hours.

-Neil