This removes the out of disk space check from CheckHealth. The disk space is now
only checked if there are files to pull, in which case pulling those files is
stopped, but everything else (dirs, links, deletes) keeps running -> can recover
disk space through pulling.
This deprecates the current minDiskFreePct setting and introduces
minDiskFree. The latter is, in it's serialized form, a string with a
unit. We accept percentages ("2.35%") and absolute values ("250 k", "12.5
Gi"). Common suffixes are understood. The config editor lets the user
enter the string, and validates it.
We still default to "1 %", but the user can change that to an absolute
value at will.
GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4087
LGTM: AudriusButkevicius, imsodin
