Set MINFREE to 80, since that's a more reasonable value according to the
literature than FFS' 90. Remove a bunch of other unused FFS cruft from
config.h.
Initialize lfs_bfree correctly vis-a-vis MIN_FREE_SEGS, so the
filesystem doesn't over-represent the amount of free space it has.
Initialize lfs_dmeta so the kernel can estimate starting from a
reasonable value.
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