Fix for `psftp-pscp-ignore-load': Default Settings is now loaded

Fix for `psftp-pscp-ignore-load': Default Settings is now loaded
before "-load" is processed so that it doesn't clobber it.

I've also changed the semantics of "-load" slightly for PSCP, PSFTP,
and Plink: if it's specified at all, it overrides (disables) the
implicit loading of session details based on a supplied hostname
elsewhere (on the grounds that the user is more likely to want the
"-load" session than the implicit session). (PuTTY itself doesn't do
implicit loading at all, so I haven't changed it.)

This means that all the PuTTY tools' behaviour is now consistent iff
"-load" is specified (otherwise, some tools have implicit-session, and
others don't).

However, I've not documented this behaviour, as there's a good chance
it will be swept away if and when we get round to sorting out how we
deal with settings from multiple sources. It's intended as a "do
something sensible" change.

[originally from svn r4352]