Some more bug fixes and just some general hygiene.
    
    Add 2 conditions to boot-success timer and service:
      Don't run it for system users
      Resolves: rhbz#1592201
      Don't run it when pkexec isn't available
      Resolves: rhbz#1619445
    Use -Wsign-compare -Wconversion -Wextra in the build.
      Related: rhbz#1624532
      Related: rhbz#1626844
    
    Signed-off-by: Peter Jones <pjones@redhat.com>