The cap file was removed for a cap of `infinity` or a RAM percentage,
so the boot check warned on every start of such an agent, for a
legitimate config. The file is now written on every start:
- a size: its byte count, as before;
- `infinity`: the literal `infinity`. The generators set no JSC heap
ceiling and no subagent MemoryHigh=, and the boot check is quiet;
- a percentage: resolved on the host the way systemd resolves
MemoryMax=N% for the container@ unit. That is permyriad (at most two
decimals, at most 100%) of sysconf(_SC_PHYS_PAGES) *
sysconf(_SC_PAGESIZE), rounded down to a page;
- a value systemd ignores (zero, over 100%, more than two decimals,
unparseable) leaves the unit uncapped, so it is `infinity`.
The file is missing only when hive-c0re can't tell the cap: the
limits file and the kept drop-in are both unreadable, or a percentage
meets an unknown host memory size. The boot check now warns only for
a missing file or a first line that is neither a byte count nor
`infinity`.
Part of #5081 (slice S0-2).
eaca3c4eb3