TODO Done. Superuser backend slot reservations

TODO item:

Administration -
    Reserve last few process slots for super-user if max_connections
    reached


Notes:

Added GUC superuser_reserved_connections such that non-superuser connections
are only acceptable in the first
(max_connections - superuser_reserved_connections) backend slots.

Superuser connections within these first n slots count towards this
non-superuser connection limit. Therefore there can be at most this number
of non-superuser connections but may be less.

In addition, this limit is only checked on initialisation of a backend
process. So reserved slots can be taken by connections that subsequently
lose superuser priviledges thus evading the lower limit on backends.

Passed regression tests, not that it was likely not to.
Behaved as expected in a manual test.


--
Nigel J. Andrews
Director

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Logictree Systems Limited
Computer Consultants