Sign of log severity values may get lost, depending on compiler

LogSeverity::Log().toInt() returns a possibly negative integer, which is cast into a char in several places, with the assumption that char's are signed by default. This assumption is not true for all platforms; GCC on ppc64 and s390x treats char's as unsigned by default. This causes the unit test server_options_test to fail on those platforms.

The fix is to explicitly declare LogComponentSettings::_minimumLoggedSeverity as a signed char array.