shmem_exit to replace exitpg().

After applying the following patch there remain two
probable buffer overruns detected by Electric Fence during
the regression test.
I'll try find out what causes the remain two ones.

This patch also corrects a typo in smgr.c.

Another PGINDENT run that changes variable indenting and case label indenting.  Also static variable indenting.

   mdunlink() and mdclose() (too !!!) now free MdfdVec for relation
   and add it to free list, so it may be re-used for another relation
   later.
2. Fix VFD-manager memory leak (found by Massimo ... and me):
   mdunlink() has to call FileUnlink() to free allocation for fileName
   and add the Vfd slot to the free list.

Add #include "postgres.h" as required

Remove #include "c.h" *and* "utils/elog.h" as applicable