a) Instead of parsing the bus messages inside of selinux-access.c
simply pass everything pre-parsed in the functions
b) implement the access checking with a macro that resolves to nothing
on non-selinux builds
c) split out the selinux checks into their own sources
selinux-util.[ch]
d) this unifies the job creation code behind the D-Bus calls
Manager.StartUnit() and Unit.Start().
Lennart Poettering