FSSplitLeft and FSSplitAbove do not really need changing window using
wincmd commands: they did already changed windows after splitting.
'b:' prefixes for variables curspr and cursb are not really needed
(and unletting them too), because they get in fact local variables in
function FSwitch(), moreover using them as local-buffer variables may
lead to error messages about inexistent variables in some specific cases
when option bufhidden is set to 'delete'.