`parse_bytes` knew only K/M/G/T, in either case. A size with the bare
`B` unit (`1048576B`), which `validate_memory_max` stores and systemd
enforces, became `infinity` in the agent's cap file: the cgroup was
capped while the agent was told there was no cap. It now takes exactly
the single-term forms systemd's `parse_size` does: no suffix, `B`, or
K/M/G/T/P/E, case-sensitive. Lowercase units and `4GB`, which systemd
refuses and so leaves the unit uncapped, read as `infinity` too.
A test runs every suffix form the validator accepts through it and
checks the stored value parses to the right byte count.
Part of #5081 (slice S0-2).
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