systemctl: avoid "leaking" some strings in UnitStatusInfo

systemctl: avoid "leaking" some strings in UnitStatusInfo

% valgrind --leak-check=full systemctl status multipathd.service --no-pager -n0
...
==431== 16 bytes in 2 blocks are definitely lost in loss record 1 of 2
==431==    at 0x4C2BBAD: malloc (vg_replace_malloc.c:299)
==431==    by 0x534AF19: strdup (in /usr/lib64/libc-2.23.so)
==431==    by 0x4E81AEE: free_and_strdup (string-util.c:794)
==431==    by 0x4EF66C1: map_basic (bus-util.c:1030)
==431==    by 0x4EF6A8E: bus_message_map_all_properties (bus-util.c:1153)
==431==    by 0x120487: show_one (systemctl.c:4672)
==431==    by 0x1218F3: show (systemctl.c:4990)
==431==    by 0x4EC359E: dispatch_verb (verbs.c:92)
==431==    by 0x12A3AE: systemctl_main (systemctl.c:7742)
==431==    by 0x12B1A8: main (systemctl.c:8011)
==431==
==431== LEAK SUMMARY:
==431==    definitely lost: 16 bytes in 2 blocks

This happens because map_basic() strdups the strings. Other code in systemctl
assigns strings to UnitStatusInfo without copying them, relying on the fact
that the message is longer lived than UnitStatusInfo. Add a helper function
that is similar to map_basic, but only accepts strings and does not copy them.
The alternative of continuing to use map_basic() but adding proper cleanup
to free fields in UnitStatusInfo seems less attractive because it'd require
changing a lot of code and doing a lot of more allocations for little gain.

(I put "leaking" in quotes, because systemctl is short lived anyway.)