Picotls fallback source

The files in this directory are a minimal OpenSSL-backed subset of Picotls,
pinned to commit:

  f07f1c8c68b237f1468bc1f1fe1b68aba3ff23b4

Upstream:

  https://github.com/h2o/picotls

This is the Picotls revision documented by ngtcp2 1.25.0.

Only the MIT-licensed Picotls TLS core and OpenSSL binding are included.
The minicrypto backend and its third-party submodules are deliberately not
included or built.

The Picotls source files retain their upstream copyright and MIT license
notices.
