feat: fix openvpn-server config sync and group settings

[davinyue]

Fix netifd config sync so LuCI changes take effect at runtime.

Add more OpenVPN server options including max_clients, duplicate_cn and uploadable certificate files.

Group settings into tabs for basic, control, push, certificate and debug sections.

Pull request details

Description

Screenshot or video of changes (if applicable)

Maintainer (preferred)

@

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Tested on

OpenWrt version:
LuCI version:
Web browser(s):

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Checklist

• This PR is not from my main or master branch 💩, but a separate branch. ✅
• Each commit has a valid ✒️ Signed-off-by: <my@email.address> row (via git commit --signoff).
• Each commit and PR title has a valid 📝 <package name>: title first line subject for packages.
• Incremented 🆙 any PKG_VERSION in the Makefile.
• (Optional) Includes what Issue it closes (e.g. openwrt/luci#issue-number).
• (Optional) Includes what it depends on (e.g. openwrt/packages#pr-number in sister repo).

[davinyue]