MEDIUM: mux-h1: Use a h1c flag to block reads when splicing is in-progress

MEDIUM: mux-h1: Use a h1c flag to block reads when splicing is in-progress

Instead of using 2 flags on the H1 stream (H1S_F_BUF_FLUSH and
H1S_F_SPLICED_DATA), we now only use one flag on the H1 connection
(H1C_F_WANT_SPLICE) to notify we want to use splicing or we are using
splicing. This flag blocks the calls to rcv_buf() connection callback.

It is a bit easier to set the H1 connection capability to receive data in
its input buffer instead of relying on the H1 stream.