mod_fix: new helper fixup function that returns string and/or int value from a gparam_t
registrar: Fix harmless compiler warning
dialplan: move the "attrs" pvar into the "dp_translate" param list

* simplifies the scripting logic
* more efficient - it is now a local option, as opposed to a global one
dispatcher: add ability to handle string PVs for ds_select_dst/domain
Fix issue introduced in commit "23174d2629c"
tm: change the "fr_(inv_)?timer_avp" params into module vars

* namely $T_fr_timer and $T_fr_inv_timer
* simplifies the scripting logic (also prevents some errors)
* less CPU cycles, less mem usage
* documentation updated