pgsql: Change ps_status.c to explicitly track the current logical length

Log Message:
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Change ps_status.c to explicitly track the current logical length of ps_buffer.
This saves cycles in get_ps_display() on many popular platforms, and more
importantly ensures that get_ps_display() will correctly return an empty
string if init_ps_display() hasn't been called yet. Per trouble report
from Ray Stell, in which log_line_prefix %i produced junk early in backend
startup.

Back-patch to 8.0. 7.4 doesn't have %i and its version of get_ps_display()
makes no pretense of avoiding pad junk anyhow.

Modified Files:
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pgsql/src/backend/utils/misc:
ps_status.c (r1.40 -> r1.41)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/ps_status.c?r1=1.40&r2=1.41)