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Harden check for submit functions

nghttp2_submit_{headers,request}: Return NGHTTP2_ERR_INVAILD_ARGUMENT
if pri_spec->type is invalid.

nghttp2_submit_push_promise: Return NGHTTP2_ERR_PROTO if issued by
client.

nghttp2_submit_altsvc: Return NGHTTP2_ERR_PROTO instead of
NGHTTP2_ERR_INVALID_STATE if issued by client.
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