dump-agent should reserve some space for interactive use

[cbiffle]

Currently, dump-agent uses the same box of RAM for recording persistent core dumps, and for temporarily reading out chunks of data to implement operations like tasks and ringbuf.

This means if a task has crashlooped and filled the core dump area, it becomes impossible to run tasks or ringbuf to find out why.

eliza@castle ~ rack:berlin $ humility -a build-gimlet-c-dev-image-default.vpd-test.zip -i 'fe80::aa40:25ff:fe04:100%berlin_sw0tp0' tasks control-plane-agent
humility: connecting to fe80::aa40:25ff:fe04:100%12
humility tasks failed: 0x24000538 can't be read via the archive or over the network

Caused by:
    out of space for dump_agent; use `humility dump --initialize-dump-agent` to free space

I think we might want to reserve some amount of dump space for this kind of interactive operation, and not use it for recording core dumps.

(Found while debugging with @hawkw)

[cbiffle]

For what it's worth humility dump --initialize-dump-agent does clear up some space very briefly