core: refactor cpu shares/blockio weight cgroup logic

core: refactor cpu shares/blockio weight cgroup logic

Let's stop using the "unsigned long" type for weights/shares, and let's
just use uint64_t for this, as that's what we expose on the bus.

Unify parsers, and always validate the range for these fields.

Correct the default blockio weight to 500, since that's what the kernel
actually uses.

When parsing the weight/shares settings from unit files accept the empty
string as a way to reset the weight/shares value. When getting it via
the bus, uniformly map (uint64_t) -1 to unset.

Open up StartupCPUShares= and StartupBlockIOWeight= to transient units.