core: support for RVEs in fparam fixups

core: support for RVEs in fparam fixups

Hack to allow using RVEs in "safe" fparam fixups. This way all the
module functions using them will automatically have var support
(most ser modules that already supported variables as quoted
 parameter, support now any variable expression, without quotes
 too).
The cleanup is done by the core (this is the "hack" part).

E.g.:
t_set_fr($v + 2 + $x)  should work now
(it uses fixup_var_int_1 which is RVE-safe).

t_set_fr("$v") is now equivalent with t_set_fr($v).