http2: edge case errors, consistency, nits

[sebdeckers]

Discovered these while working with everyone's new favourite API, res.respondWithFile. 😉

• Cleaned up a stray console.log in test-http2-respond-file.js. /cc @jasnell
• Made the compat res.end(cb) callback behave the same as http1 (only fires the first time). Also no errors thrown on repeated res.end() calls.
• Fixed errors when calling res.stream.respondWithFile from the compatibility layer. Seems like a valid use-case (well, I'm using it 🤣).

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

http2

[sebdeckers]

Pushed a few more changes.

• Achieves the same result without touching stream._readableState.
• Additional test case for HEAD requests at the compat response layer (was breaking with the earlier patch).
• Expose the internal flag state.headersSent as ServerHttp2Stream.headersSent to signal whether or not the .respond() methods can still be called. Earlier this was tracked separately by Http2ServerResponse.headersSent but that went out of sync now that HEAD requests and FD's are handled directly by the stream.
• Updated the var declarations to const and let where appropriate. Sorry about the lines changed noise.

[sebdeckers]

Added:

• http2.connect() documentation shows that options and listener are optional. Added test cases for this.
• Log output consistency fix to put the session type prefix in square brackets.

[sebdeckers]

@jasnell De-linted. Welcome back! 👋

[jasnell]

Landed!