Create an fd.c entry point that is just like plain open(2) except that

it will close VFDs if necessary to surmount ENFILE or EMFILE failures.
Make use of this in md.c, xlog.c, and user.c routines that were
formerly vulnerable to these failures.  In particular, this should
handle failures of mdblindwrt() that have been observed under heavy
load conditions.  (By golly, every other process on the system may
crash after Postgres eats up all the kernel FDs, but Postgres will
keep going!)