BUG/MINOR: mux-h1: Save shutdown mode if the shutdown is delayed

BUG/MINOR: mux-h1: Save shutdown mode if the shutdown is delayed

The connection shutdown may be delayed if there are pending outgoing
data. The action is performed once data are fully sent. In this case the
mode (dirty/clean) was lost and a clean shutdown was always performed. Now,
the mode is saved to be sure to perform the connection shutdown using the
right mode. To do so, H1C_F_ST_SILENT_SHUT flag is introduced.

This patch should be backported as far as 2.0.