Bugfix and improvements in multixact.c

Hi!

While working on a making multix xact offsets 64-bit [0] I've discovered a minor issue.  The 

thing is that type 'xid' is used in all macro, but it doesn't correct.  Appropriate MultiXactId or 

MultiXactOffset should be used, actually.

And the second thing, as Heikki Linnakangas points out, args naming is also misleading.

Since, these problems are not the point of thread [0], I decided to create this discussion.
And here is the patch set addressing mentioned issues (0001 and 0002).

Additionally, I made an optional patch 0003 to switch from macro to inline functions.  For 

me, personally, use of macro functions is justified if we are dealing with different argument 

types, to make polymorphic call.  Which is not the case here.  So, we can have more 

control over types and still generate the same code in terms of speed.  

See https://godbolt.org/z/KM8voadhs Starting from O1 function is inlined, thus no 

overhead is noticeable.  Anyway, it's up to the actual commiter to decide does it worth it

or not.  Again, this particular patch 0003 is completely optional.

As always, any opinions and reviews are very welcome!

[0] https://www.postgresql.org/message-id/flat/ff143b24-a093-40da-9833-d36b83726bdf%40iki.fi#61d5a0e1cf6ab94b0e8aae8559bc4cf7