bus: drop creds->capability_size

The number of available caps can be read from
/proc/sys/kernel/cap_last_cap during runtime. Our helper cap_last_cap()
does that, so there's no reason to remember the size of any capability
cache. We can just pre-allocate arrays with a suitable size for all
available caps and reject any higher caps.

The kernel capability API uses u32 as base so make sure we do the same.
Note that this is specified by POSIX, so it's unlikely to change.
bus: fix capabilities on big-endian

The kernel provides capabilities as a u32 array, sd-bus uses an u8 array.
This works fine on little-endian as both are encoded the same way.
However, this fails on big-endian if we do not perform sufficient
byte-swapping on each u32 entry.

This patch makes sd-bus use u32, too. We avoid changing any kernel
provided data so we can keep pointing into kdbus pool buffers which
contain u32 arrays.