Share-policy plugin: extension blocklist + size/file-count thresholds

[Aybook]

Extend the existing usr_min_share.lua and similar share-policy scripts.

Scope

• Extension blocklist + warn-list (*.lnk, *.url, *.scr, *.bat, *.cmd, *.ps1, *.vbs, thumbs.db, desktop.ini, ...). Cfg-tunable.
• Minimum file count (separate from minimum byte size).
• Empty-share rejection.
• Sane default: small block list of obvious junk; everything else off until operator enables.

Out of scope

• Heuristic fake/junk detection.
• Bulk filelist scanning at login (perf-prohibitive).

Open questions

• Hub-side enumeration: do we have access to filenames at login (BINF flags) or only at search time (BSCH)?

Acceptance

• Default cfg blocks nothing.
• Operator-enabled blocks: client login with *.lnk in share -> kicked / warned per cfg.