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

When syslog-ng is started for checking a config file,
control socket is "stolen" by the syntax-checker syslog-ng
from a previously started syslog-ng.

References: #276

Signed-off-by: Budai Laszlo <Laszlo.Budai@balabit.com>
Merge pull request #277 from lbudai/3.6/rcbugs

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