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>
