pgsql: Set MaxBackends only on bootstrap and standalone modes

Set MaxBackends only on bootstrap and standalone modes

... not on auxiliary processes.  I managed to overlook the fact that I
had disabled assertions on my HEAD checkout long ago.

Hopefully this will turn the buildfarm green again, and put an end to
today's silliness.

Branch
------
master

Details
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http://git.postgresql.org/pg/commitdiff/15658911d9947cda031a42bcbc3e2b745dff8f49

Modified Files
--------------
src/backend/tcop/postgres.c       |    5 +++++
src/backend/utils/init/postinit.c |    2 --
2 files changed, 5 insertions(+), 2 deletions(-)