pgsql: Move SQL-callable code related to multixacts into its own file

Move SQL-callable code related to multixacts into its own file

A patch is under discussion to add more SQL capabilities related to
multixacts, and this move avoids bloating the file more than necessary.
This affects pg_get_multixact_members(). A side effect of this move is
the requirement to add mxstatus_to_string() to multixact.h.

Extracted from a larger patch by the same author, tweaked by me.

Author: Naga Appani <nagnrik(at)gmail(dot)com>
Reviewed-by: Michael Paquier <michael(at)paquier(dot)xyz>
Reviewed-by: Ashutosh Bapat <ashutosh(dot)bapat(dot)oss(at)gmail(dot)com>
Discussion: https://postgr.es/m/CA+QeY+AAsYK6WvBW4qYzHz4bahHycDAY_q5ECmHkEV_eB9ckzg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/df9133fa63843627fb3579c89d892dc3d9ea2b95

Modified Files
--------------
src/backend/access/transam/multixact.c | 66 +-------------------------
src/backend/utils/adt/Makefile | 1 +
src/backend/utils/adt/meson.build | 1 +
src/backend/utils/adt/multixactfuncs.c | 87 ++++++++++++++++++++++++++++++++++
src/include/access/multixact.h | 1 +
5 files changed, 91 insertions(+), 65 deletions(-)