transaction: when verifying transaction destructiveness, check if previous job is superset of new
job: special case JOB_NOP to avoid hitting assertion failures, remove two redundant checks
execute: indent cases in setup_output()
units: get rid of unneeded syslog.socket

We don't hijack /dev/log and force syslog daemons to listen on
the journal socket, since we do not have the journal. As such,
this is safe to remove.
man: remove references to syslog.socket
control-request: add preset
control-request: add link
control-request: add is-enabled
control-request: add disable
control-request: fix strerror()s
control-request: fix missing error checks
control-request: add enable
control-request: add reenable
control-request: print plain string value when asking for default target