pgsql: Prevent psql from issuing BEGIN before ALTER SYSTEM when AUTOCOM

Prevent psql from issuing BEGIN before ALTER SYSTEM when AUTOCOMMIT is off.

The autocommit-off mode works by issuing an implicit BEGIN just before
any command that is not already in a transaction block and is not itself
a BEGIN or other transaction-control command, nor a command that
cannot be executed inside a transaction block. This commit prevents psql
from issuing such an implicit BEGIN before ALTER SYSTEM because it's
not allowed inside a transaction block.

Backpatch to 9.4 where ALTER SYSTEM was added.

Report by Feike Steenbergen

Branch
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REL9_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/5520006b5bbaa36e6734a8ccf960bb870f4a4fec

Modified Files
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src/bin/psql/common.c |   17 +++++++++++++++++
1 file changed, 17 insertions(+)