pgsql: pg_upgrade: Read nextMultiOffset as a 64-bit value.

pg_upgrade: Read nextMultiOffset as a 64-bit value.

Commit bd8d9c9bdfa widened MultiXactOffset to 64 bits and widened
ControlData.chkpnt_nxtmxoff accordingly, but get_control_data() still
read the "Latest checkpoint's NextMultiOffset" line with str2uint(),
which returns unsigned int.

This commit adds str2uint64(), mirroring the str2uint() helper used
for the other control file fields, and reads the offset with it.

Backpatch to v19, where MultiXactOffset was widened.

Reviewed-by: Heikki Linnakangas <hlinnaka(at)iki(dot)fi>
Reviewed-by: Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAD21AoCvzerscfU8o4ARQ793yAGHpQ72r2x5apeC_W2-k=SLCQ@mail.gmail.com
Backpatch-through: 19

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/058962fdc98ea05968cd18f7b87610f9f2dd28d2

Modified Files
--------------
src/bin/pg_upgrade/controldata.c | 2 +-
src/bin/pg_upgrade/pg_upgrade.h | 1 +
src/bin/pg_upgrade/util.c | 11 +++++++++++
3 files changed, 13 insertions(+), 1 deletion(-)