guile: fix crash on calls to callbacks during load of script (bug #38343)
guile: fix arguments given to callbacks (separate arguments instead of one list with arguments inside), guile >= 2.0 is now required (bug #38350)

Unfortunately this breaks compatibility with existing guile scripts, but this
change is needed to keep consistency with other scripting languages.

Now guile >= 2.0 is required, WeeChat can not compile any more with guile 1.8.