MINOR: mux-h2: make h2c_decode_headers() return a status, not a count

MINOR: mux-h2: make h2c_decode_headers() return a status, not a count

This function used to return a byte count for the output produced, or
zero on failure. Not only this value is not used differently than a
boolean, but it prevents us from returning stream errors when a frame
cannot be extracted because it's too large, or from parsing a frame
and producing nothing on output.

This patch modifies its API to return <0 on errors, 0 on inability to
proceed, or >0 on success, irrelevant to the amount of output data.