exec: Add SELinuxContext configuration item

This permit to let system administrators decide of the domain of a service.
This can be used with templated units to have each service in a différent
domain ( for example, a per customer database, using MLS or anything ),
or can be used to force a non selinux enabled system (jvm, erlang, etc)
to start in a different domain for each service.

src/core/execute.h

@@ -133,6 +133,8 @@ struct ExecContext {
 
         char *utmp_id;
 
+        char *selinux_context;
+
         char **read_write_dirs, **read_only_dirs, **inaccessible_dirs;
         unsigned long mount_flags;