Log file path now references SDP_INSTANCE environment variable
if defined (the Windows SDP default), else P4INSTANCE (the Unix
SDP defeault).
Changed from hard-coded 'perforce' user to reference P4USER if
defined, else falling back to 'perforce' as a default.
Fixed issue of differeing observed Python behavior when attempting
to return an undefined field value. It seems Python on my Linux
system defaults to 'None', while Python on my Windows system
throws an exception (or just dies). Not 100% sure this is a
Python on Windows/Python on Linux difference, or maybe slightly
different Python 3.3 verions? In any case, the fix was improved
defensive coding.