Re: CustomScan support on readfuncs.c

Hi,

I tried to define two additional callbacks to support CustomScan
on readfuncs.c.

First of all, we need to pay attention how to treat output of
TextOutCustomScan when additional text output is generated.
Only custom-scan provider knows what shall be displayed, so
all we can do is invoke a new callback: TextReadCustomScan().
Because private fields shall be displayed next to the common
field of CustomScan, this callback is invoked once pg_strtok
pointer reached to the last field of CustomScan. Then, custom
scan provider allocates CustomScan or a structure embedding
CustomScan; only CSP knows exact size to be allocated.
It can fetch some tokens using pg_strtok and reconstruct its
private fields, but no need to restore the common field because
it shall be done by _readCustomScan.