channels: allow channel_open() to return SSH_AGAIN
channels: allow channel_open() to return SSH_AGAIN

Signed-off-by: Nicolas Viennot <418d940643b1975d62234ee01246ad4b58904184@viennot.biz>
Reviewed-by: Andreas Schneider <5be00ddc76278cf6077f5047ca3384a88460c671@cryptomilk.org>

taikoo/libssh,mwgoldsmith/libssh,pouete/libssh,DouglasHeriot/libssh,sebadoom/libssh,kedazo/libssh,pouete/libssh,bigcat26/libssh-mod,robxu9/libssh,mwgoldsmith/libssh,wangshawn/libssh,taikoo/libssh,Distrotech/libssh,wangshawn/libssh,bigcat26/libssh-mod,elastichosts/libssh,mwgoldsmith/ssh,kedazo/libssh,mwgoldsmith/ssh,elastichosts/libssh,bigcat26/libssh-mod,mwgoldsmith/libssh,mwgoldsmith/ssh,pouete/libssh,nviennot/libssh,nviennot/libssh,wangshawn/libssh,pouete/libssh,Distrotech/libssh,bigcat26/libssh-mod,mwgoldsmith/ssh,sebadoom/libssh,Distrotech/libssh,taikoo/libssh,DouglasHeriot/libssh,robxu9/libssh,robxu9/libssh,DouglasHeriot/libssh,DouglasHeriot/libssh,nviennot/libssh,kedazo/libssh,nviennot/libssh,Distrotech/libssh,wangshawn/libssh,kedazo/libssh,robxu9/libssh,elastichosts/libssh,sebadoom/libssh,sebadoom/libssh,mwgoldsmith/libssh,taikoo/libssh