bpo-33773: Fix test.support.fd_count() on Linux/FreBSD (GH-7421)

Substract one because listdir() opens internally a file
descriptor to list the content of the /proc/self/fd/ directory.

Add test_support.test_fd_count().

Move also MAXFD code before msvcrt.CrtSetReportMode(), to make sure
that the report mode is always restored on failure.