service: properly remember if a sysv is actually enabled

service: properly remember if a sysv is actually enabled

Previously we checked the SysV priority value to figure out if a SysV
unit was enabled or not, since th value was mostly read from the S
startup links. Since we read this value from the LSB headers as a
fallback we hence ended up considering a lot more services enabled than
were actually enabled.

This patch adds an explicit boolean which encodes whether a sysv service
is enabled or not via S links.

https://bugzilla.redhat.com/show_bug.cgi?id=615293