pgsql: Fix count_usable_fds() to stop trying to open files once it

Log Message:
-----------
Fix count_usable_fds() to stop trying to open files once it reaches
max_files_per_process. Going further than that is just a waste of
cycles, and it seems that current Cygwin does not cope gracefully
with deliberately running the system out of FDs. Per Andrew Dunstan.

Tags:
----
REL8_0_STABLE

Modified Files:
--------------
pgsql/src/backend/storage/file:
fd.c (r1.115 -> r1.115.4.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/file/fd.c.diff?r1=1.115&r2=1.115.4.1)