main: do not intialize mainloop when only syntax-check is required

When syslog-ng is started for checking a config file,
control socket is "stolen" by main_loop_init() from a previously
started syslog-ng.

References: #276

Signed-off-by: Budai Laszlo <Laszlo.Budai@balabit.com>
Are you sure this way we init all preconditions?