Fix to #575, http://bugs.mariadb.com/show_bug.cgi?id=575
log_manager.cc:
	Log manager handles cases where there are mismatch in user privileges.
	Mark log files enabled in the global lm_enabled_logfiles_bitmask after initialization so that it reflects reality in error cases. In general, take into account the possibility that any phase in initialization may fail and read return values.
	Replaced file_exists_and_is_writable to check_file_and_path which has a slightly different logic and which detects if file open fails for a few different reasons.
	Improved logging (in stderr) in general in error cases.

gateway.c: Also check home directory accessibility in case when it is provided as a command-line argument. Added function check_dir_access to provide that function. Read return value of skysql_logmanager_init and exit (nicely) if it failed.

skygw_utils.cc: initialize mlist with version number 2, which indicates that object is initialized (different than zero) and that there are no active updates on the object (version%2==0).