information about where to put items that aren't mentioned in the
saved configuration. So far the only nontrivial use I've made of this
facility is to default to placing KEX_RSA just above KEX_WARN in the
absence of any other information, which should fix
'ssh2-rsa-kex-pref'.

While I'm here I've rewritten wprefs() on general principles to remove
the needless length limit, since I was touching it anyway. The length
limit is still in gprefs (but I've lengthened it just in case).

[originally from svn r9181]