Fix EXSLT functions returning RVTs from outer scopes

The RVTs referenced from function results must not be blindly registered
as local, as they might be part of variables from an outer scope. Remove
LOCAL/VARIABLE distinction for RVTs. Don't register as local RVT
unconditionally when reflagging as LOCAL. Instead, register function
result RVTs from inner variables as local RVTs when they're released in
xsltFreeStackElem. Keep local function result RVTs xsltReleaseLocalRVTs
instead of reregistering.

Closes: https://gitlab.gnome.org/GNOME/libxslt/issues/2

Thanks to Daniel Mendler and Martin Gieseking for the reports.