Convert confusing macros in multixact.c to static inline functions

Convert confusing macros in multixact.c to static inline functions

The macros were confused about the argument data types. All the
arguments were called 'xid', and some of the macros included casts to
TransactionId, even though the arguments were actually either
MultiXactIds or MultiXactOffsets. It compiles to the same thing,
because TransactionId, MultiXactId and MultiXactOffset are all
typedefs of uint32, but it was highly misleading.

Author: Maxim Orlov <orlovmg@gmail.com>
Discussion: https://www.postgresql.org/message-id/CACG%3DezbLUG-OD1osAW3OchOMxZtdxHh2itYR9Zhh-a13wEBEQw%40mail.gmail.com
Discussion: https://www.postgresql.org/message-id/ff143b24-a093-40da-9833-d36b83726bdf%40iki.fi