net-p2p/qbittorrent: Adjust default settings when using libtorrent-rasterbar 2.x

• Disable memmory mapped file handling and use "POSIX-compliant"
• Backport commit 8bcac1bed28f93c0712e915f43294b1e5fd03659 which reduces FilePoolSize
• Change status to experimental

This change only applies to new installs, if you have a configuration
already you need to apply these changes by hand

References:
https://github.com/qbittorrent/qBittorrent/commit/8bcac1bed28f93c0712e915f43294b1e5fd03659
https://github.com/arvidn/libtorrent/issues/6667#issuecomment-1502670412

PR: 270765
Reviewed by: yuri (previous revision)