vfs: keep the open-file cache below RLIMIT_NOFILE on Unix

The VFS cache allowed up to 8192 open files on Unix, and main() set the
soft RLIMIT_NOFILE to exactly 8192. With large loose romfs dumps (e.g.
~205k files) the cache could exhaust every descriptor, so unrelated
opens failed with EMFILE and games crashed on the failed loads.

Size the cache from the actual soft limit with 512 descriptors spare,
as is already done on Windows, and only raise the limit in main()
instead of also lowering higher limits down to 8192.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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