Remove the TAP backend: userspace proxy only (2.0.0) #72

Why. vtesse is a VTun client that needs no root and no kernel interface: lwIP, SOCKS5, port forwards, Android. The TAP backend (--tap) is a second, different product inside it — root, /dev/net/tun, hook and DHCP scripts — and doubles the build matrix and the tests. A kernel-interface client for stock vtund servers (type tun, tun.ko or camex.ko) moves to a separate project.

Change.

• Remove netcore/tapdev.c, include/netcore/tapdev.h, the TAP build option and LWIP=0, --tap, --ifname, --hook.
• Remove scripts/vtesse-hook.sh, scripts/vtesse-udhcpc.script, the built-in copies of both and the TAP parts of --install.
• CI: drop the TAP-only build and bench.sh tap-test (and the TAP half of tun-test); the release loses the two scripts.
• --tap, tap= in /etc/vtesse.conf or a hook key: a clear error naming the change, so an old installation fails loudly instead of silently changing mode.
• README (en, ru, zh-CN), docs/usage.md, docs/architecture.md, CHANGELOG.
• Both type ether (DHCP) and type tun (ip=) stay in the lwIP backend.

Version. Incompatible: 2.0.0, released together with the default-route wait (separate issue, separate PR).

Related: #68 (narrowed to lwIP), #69 (moves out).