core: when logging about dependency cycles, add UNIT= entries for all involved units

core: when logging about dependency cycles, add UNIT= entries for all involved units

Example log:
Jul 22 15:55:21 fedora systemd[1]: a1.service: Found ordering cycle on a2.service/start
Jul 22 15:55:21 fedora systemd[1]: a1.service: Found dependency on a3.service/start
Jul 22 15:55:21 fedora systemd[1]: a1.service: Found dependency on a1.service/start
Jul 22 15:55:21 fedora systemd[1]: a1.service: Job a2.service/start deleted to break ordering cycle starting with a1.service/start
Jul 22 15:55:21 fedora systemd[1]: Starting a1.service...
Jul 22 15:55:21 fedora systemd[1]: Started a1.service.

Example log entry:

Sat 2017-07-22 15:55:21.372389 EDT [s=0004bb6302d94ac3aa69987fb6157338;i=9ae;b=a96eb6153d4f4f3686c7b4
    _BOOT_ID=a96eb6153d4f4f3686c7b4db8a432908
    _MACHINE_ID=ad18f69b80264b52bb3b766240742383
    _HOSTNAME=fedora
    PRIORITY=3
    SYSLOG_FACILITY=3
    SYSLOG_IDENTIFIER=systemd
    _UID=0
    _GID=0
    _PID=1
    _TRANSPORT=journal
    _CAP_EFFECTIVE=3fffffffff
    _COMM=systemd
    _EXE=/usr/lib/systemd/systemd
    _SYSTEMD_CGROUP=/init.scope
    _SYSTEMD_UNIT=init.scope
    _SYSTEMD_SLICE=-.slice
    _SELINUX_CONTEXT=system_u:system_r:kernel_t:s0
    CODE_FILE=../src/core/transaction.c
    CODE_FUNC=transaction_verify_order_one
    UNIT=a3.service
    UNIT=a1.service
    UNIT=a2.service
    CODE_LINE=430
    MESSAGE=a1.service: Job a2.service/start deleted to break ordering cycle starting with a1.service
    _CMDLINE=/usr/lib/systemd/systemd --system --deserialize 28
    _SOURCE_REALTIME_TIMESTAMP=1500753321372389

This should make it easier to see when any of the units are involved in an
ordering cycle.

Fixes #6336.

v2:
- also update the "Unable to break cycle" message.