Apologies for repeating what toaster said a little, I feel more has to be clarified on what the functions do and why you don't need to have access to them.
P_SetTarget is a function to do a mobj pointer assignment (usually mobj.target, but is also used for mobj.tracer and other points), while also marking it as a "reference" to the mobj (or removing the reference if it's assigned to a new mobj or nothing), so that when the mobj is removed, it doesn't immediately go away until all "references" to it have been cleared. This prevents certain problems from happening with pointers to mobjs that no longer exist. In Lua, the function is automatically used whenever you assign a new value to mobj.target or mobj.tracer and a bunch of other already existing mobj pointers. Custom Lua variables set to mobj_t meanwhile are automatically cleaned up if said mobjs no longer exist (I think). So you shouldn't need access to that function.
P_UnsetThingPosition and P_SetThingPosition are functions to unlink an object from the blockmap and sector lists, and relink it to them using the current position and flags. Therefore in the game it's only used if the x and y positions were modified, or if you changed whether an object uses the MF_NOBLOCKMAP or MF_NOSECTOR flags or not. Lua only allows you to change x/y using functions like P_TeleportMove/P_TryMove, and automatically uses the functions if you turn on or off those two flags. So you don't need access to those two functions either.