Create a GUC parameter max_files_per_process that is a configurable

Create a GUC parameter max_files_per_process that is a configurable
upper limit on what we will believe from sysconf(_SC_OPEN_MAX).  The
default value is 1000, so that under ordinary conditions it won't
affect the behavior.  But on platforms where the kernel promises far
more than it can deliver, this can be used to prevent running out of
file descriptors.  See numerous past discussions, eg, pgsql-hackers
around 23-Dec-2000.