pgsql: Fix Y2038 issues with MyStartTime.

Fix Y2038 issues with MyStartTime.

Several places treat MyStartTime as a "long", which is only 32 bits
wide on some platforms.  In reality, MyStartTime is a pg_time_t,
i.e., a signed 64-bit integer.  This will lead to interesting bugs
on the aforementioned systems in 2038 when signed 32-bit integers
are no longer sufficient to store Unix time (e.g., "pg_ctl start"
hanging).  To fix, ensure that MyStartTime is handled as a 64-bit
value everywhere.  (Of course, users will need to ensure that
time_t is 64 bits wide on their system, too.)

Co-authored-by: Max Johnson
Discussion: https://postgr.es/m/CO1PR07MB905262E8AC270FAAACED66008D682%40CO1PR07MB9052.namprd07.prod.outlook.com
Backpatch-through: 12

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/c91d0af0a2ad3d3176eaf508281a4e29bb1975b8

Modified Files
--------------
src/backend/utils/error/elog.c    | 8 ++++----
src/backend/utils/init/miscinit.c | 4 ++--
src/bin/pg_ctl/pg_ctl.c           | 2 +-
3 files changed, 7 insertions(+), 7 deletions(-)