http2: Don't send header fields prohibited by HTTP/2 spec

[tatsuhiro-t]

Previously, we just ignored "Connection" header field. But HTTP/2
specification actually prohibits few more header fields. This commit
ignores all of them so that we don't send these bad header fields.

[mention-bot]

@tatsuhiro-t, thanks for your PR! By analyzing the history of the files in this pull request, we identified @Andersbakken, @jay and @bagder to be potential reviewers.

[jay]

trailers can be anywhere so I think we will have to account for this, for example
TE: trailers, deflate
TE: deflate, trailers;qabc

[tatsuhiro-t]

Good point.. Yes, we need to parse it. Does curl have convenient function to parse this comma separated list header fields?

[jay]

Does curl have convenient function to parse this comma separated list header fields?

hm offhand I can't think of one. There's always strtok_r. Also I don't think we have to check for the semi-colon in trailers if I'm reading this right:

       TE        = "TE" ":" #( t-codings )
       t-codings = "trailers" | ( transfer-extension [ accept-params ] )
       transfer-coding         = "chunked" | transfer-extension
       transfer-extension      = token *( ";" parameter )

[tatsuhiro-t]

Thank you. I ended up not to use strtok_r since it requires NULL-terminated string, and it replaces delimiter with NUL byte. Instead, I added hand crafted "trailers" detection code.
Conflict has been resolved as well.

[jay]

I ended up not to use strtok_r since it requires NULL-terminated string, and it replaces delimiter with NUL byte. Instead, I added hand crafted "trailers" detection code.

Oops I forgot about that. Your changes just landed. I made a few corrections:

• Typo. Searching for tab had used 't' instead of '\t'
• Unnecessary goto. We discourage goto when not necessary so I redid the logic.
• enum names too generic. (For example IGNORE is defined to 0 somewhere in my includes so I prefixed them with HEADERINST_)
• Unreachable code. Some compilers may warn that since the for(;;) is never exited the return after that block will never be hit, so I removed it.
• Minor style changes

Thanks!

[tatsuhiro-t]

The commit looks good. Thank you for merging this.