MINOR: h1: h1_release() should return if it destroyed the connection

MINOR: h1: h1_release() should return if it destroyed the connection

h1_release() is called to destroy everything related to the mux h1,
usually even the connection. However, it handles upgrades to HTTP/2 too,
in which case the h1 mux will be destroyed, but the connection will
still be alive. So make it so it returns 0 if everything is destroyed,
and -1 if the connection is still alive.

This should be backported up to 2.8, as a future bugfix will depend on
it.