lcm-logger: New --disk-quota flag

[judfs]

Fix #464

      --disk-quota=SIZE      Minimum amount of free space the disk written to
                             must maintain. lcm-logger will exit if the quota is
                             exceeded. This is NOT an explicit limit on how much
                             lcm-logger is allowed to write.
                             Units accepted: B
                             1024: K,   M,   G,   T,   P,   E,
                                   Ki,  Mi,  Gi,  Ti,  Pi,  Ei,
                                   KiB, MiB, GiB, TiB, PiB, EiB
                             1000: KB, MB, GB, TB, PB, EB
                             e.g. "50G" or "0.05 TiB". 
                             Note: for `df`, -h is 1024 and -H is 1000 units.
                             Units are treated case insensitively. gb is treated
                             as GB (gigabytes) and not as gigabits.
                             NOT currently supported on Windows!

Currently put the check in the write thread and does not cause an immediate exit(1). Please review.

[nosracd]

I think the current approach is ideal. If the user sets up a condition for the logger to end then a non-zero exit code makes sense to me, since everything is operating as expected. But I don't feel strongly at all about it.

[nosracd]

Looks good to me. Just a couple notes:

• There are still a few@Review in-code comments, but I believe I already replied to them or they appear to be notes to self. Can they be removed?
• There's also a man pages document (lcm-logger/lcm-logger.1) that should be updated when the UX changes. This wouldn't be the first thing that caused those documents to go out of sync so I think it's fine to fix it either here or later in Update man pages #537.