syslog-lib: preserve SystemLogSocketName setting

As noted in rsyslog commit 3b55048094d759adf8162f0829f239e115ebf2c0,
rsyslog by default deletes log socket during restart, unless socket
is provided by systemd.

This testcase is using rsyslog config, which does not specify
any $SystemLogSocketName, so rsyslog uses default value "/dev/log".
This creates problem on distros using journald, because "/dev/log"
is owned by journald and rsyslog removes it.

Test works normally, but after it completes, /dev/log
stays removed even after rsyslog config is restored - distro
config defines log socket as "/run/systemd/journal/syslog".

This patch includes "$SystemLogSocketName" line from rsyslog
distro configuration (if present) to rsyslog.conf used in
LTP syslog* tests.

Distros affected by this issue (e.g. RHEL7.0) will now rely on
journald to forward messages to "$SystemLogSocketName", where
rsyslog can find them:
  syslog(2) -> /dev/log -> journald -> /run/systemd/journal/syslog -> rsyslog

Distros which don't define $SystemLogSocketName will be unaffected,
only empty line is inserted to rsyslog config.

Signed-off-by: Jan Stancek <jstancek@redhat.com>
Acked-by: Cyril Hrubis <chrubis@suse.cz>