Stop syslog.socket when entering emergency mode (#3130)

When enabling ForwardToSyslog=yes, the syslog.socket is active when entering
emergency mode. Any log message then triggers the start of rsyslog.service (or
other implementation) along with its dependencies such as local-fs.target and
sysinit.target. As these might fail themselves (e. g. faulty /etc/fstab), this
breaks the emergency mode.

This causes syslog.socket to fail with "Failed to queue service startup job:
Transition is destructive".

Add Conflicts=syslog.socket to emergency.service to make sure the socket is
stopped when emergency.service is started.

Fixes #266