python - lo: Align PGSERVICE handling with libpq.

Log Message:
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Align PGSERVICE handling with libpq.

Formerly, if a service went unfound, ConfigParser would raise a
NoSectionError. This was done regardless of the file existence or readability,
which is inconsistent with libpq's error on access, but not on undefined service.
In the future, we will probably throw a warning about a missing service.

Additionally, allow users to specify the environment dictionary to get the
PGSYSCONFDIR variable from when using service options.

Modified Files:
--------------
lo/postgresql/utility/client:
environ.py (r1.1 -> r1.2)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/lo/postgresql/utility/client/environ.py.diff?r1=1.1&r2=1.2)
option.py (r1.9 -> r1.10)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/lo/postgresql/utility/client/option.py.diff?r1=1.9&r2=1.10)