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