This commit makes it possible to configure QGIS Server to make it
write its logs to stderr.
This is done by setting the QGIS_SERVER_LOG_FILE env var to the
special value "stderr".
message log critical/warning messages
These messages are still logged, but the log indicator is not
modified to flag their appearance.
This allows certain error messages to be logged (e.g. on startup)
which cannot be fixed by users/developers (e.g. those caused
by upstream Qt issues, 3rd party Python libraries, etc) but
without flagging them.
Currently, we get so many of these warnings on startup that the
log indicator loses all impact, and newly introduced, VALID
errors go unnoticed/ignored.
