pgstat cleanup: use palloc and AllocateFile

This patch changes pgstat.c to use palloc(), AllocateFile() and
FreeFile() rather than malloc(), fopen() and fclose(), respectively.

I changed more_tabstat_space() (which is invoked at various times
indirectly throughout the backend) to allocate memory in its own private
memory context, rather than use malloc() -- we can't just use
CurrentMemoryContext because that may not be sufficiently long-lived.

Barring any objections I intend to apply this to HEAD tomorrow.

-Neil