Rearrange cpluspluscheck to check just one .h file at a time.

This is slower than the original coding but avoids the problem of
including files in an unpredictable order.  Aside from being more
trustworthy, we can get rid of some exclusions that were formerly
made for what turn out to be ordering or re-inclusion problems.

I also modified it to include libpq's exported files in the check.
ecpg should be included as well, but I'm unclear on which ecpg .h
files are meant to be included by clients.