udev: destroy manager before cleaning environment

Due to our _cleanup_ usage for the udev manager, it will be destroyed
after the "exit:" label has finished. Therefore, it is the last
destruction done in main(). This has two side-effects:
  - mac_selinux is destroyed before the udev manager is, possible causing
    use-after-free if the manager-cleanup accesses selinux data
  - log_close() is called *before* the manager is destroyed, possibly
    re-opening the log if you use --debug (and thus not re-applying the
    --debug option)

Avoid this by moving the manager-handling into a new function called
run(). This function will be left before we enter the "exit:" label in
main(), hence, the manager object will be destroyed early.