core: make SELinux enable/disable check symmetric

core: make SELinux enable/disable check symmetric

We'd use the generic check for disable, and a unit-file-specific one for enable.
Use the more specific one both ways.

systemd[1]: SELinux access check scon=system_u:system_r:systemd_timedated_t:s0 tcon=system_u:system_r:init_t:s0 tclass=system perm=disable path=(null) cmdline=/usr/lib/systemd/systemd-timedated: -13
systemd[1]: SELinux access check scon=system_u:system_r:systemd_timedated_t:s0 tcon=system_u:object_r:systemd_unit_file_t:s0 tclass=service perm=enable path=/usr/lib/systemd/system/systemd-timesyncd.service cmdline=/usr/lib/systemd/systemd-timedated: -13

https://bugzilla.redhat.com/show_bug.cgi?id=1014315