Adds --general_log_file, --slow_query_log_file command-
line options to match system variables of the same names.

Deprecates --log, --log-slow-queries command-line options
and log, log_slow_queries system-variables for v7.0; they
are superseded by general_log/general_log_file and
slow_query_log/slow_query_log_file, respectively.