New SSH bug flag, for 'can't handle SSH2_MSG_IGNORE'. Another user

New SSH bug flag, for 'can't handle SSH2_MSG_IGNORE'. Another user
today reported an SSH2_MSG_UNIMPLEMENTED from a Cisco router which
looks as if it was triggered by SSH2_MSG_IGNORE, so I'm
experimentally putting this flag in. Currently must be manually
enabled, though if it turns out to solve the user's problem then
I'll probably add at least one version string...

[Edited commit message: actually, I also committed in error a piece
of experimental code as part of this checkin. Serve me right for not
running 'svn diff' first.]

[originally from svn r8926]