Manually limit file descriptors that we select on to FD_SETSIZE.

Manually limit file descriptors that we select on to FD_SETSIZE.

This works even if another thread or process resets the resource limit
for open file descriptors, e.g., using 'prlimit' on GNU/Linux.

* src/process.c (create_process, create_pty, Fmake_pipe_process)
(Fmake_serial_process, connect_network_socket)
(server_accept_connection): Limit file descriptors to FD_SETSIZE.
* test/src/process-tests.el (process-tests--with-raised-rlimit): New
helper macro.
(process-tests--fd-setsize-test): Rename from
'process-tests--with-many-pipes'.  Increase resource limit during test
if possible.
(process-tests/fd-setsize-no-crash/make-process)
(process-tests/fd-setsize-no-crash/make-pipe-process)
(process-tests/fd-setsize-no-crash/make-network-process)
(process-tests--new-pty): Rename callers.