df: implement it

[metif12]

df for the disk-space listing: per-volume capacity, used and available space, a percentage, the file-system type and the mount point.

The presentation layer lives in common/df and src/df is an eleven-line main, because v test <dir> only compiles _test.v files, so a test in src/df cannot see code defined in src/df. Moving the logic into a module made the column layout assertable directly instead of only through the built binary.

Backends:

• Windows: GetLogicalDrives, GetDiskFreeSpaceExA and GetVolumeInformationA. Device name is the drive letter, since that is the name a Windows user has for the volume.
• Unix: /proc/mounts plus os.disk_usage (statvfs), including the kernel's octal-escape decoding for device and mount-point names.

Options: -a -B -h -H -i -k -l -m -g -P -T -t -x --total, file operands. Clustered short options (-hT), -B512 and comma-separated --type=a,b are parsed here rather than by common.flag_parser, which does not expand clusters.

Measured against GNU 9.4 before writing the code:

• Column widths are the widest of the header, the data, and a minimum GNU reserves: device 14, Size/Used 5, Use% 4. The last column is not padded, so there are no trailing spaces.
• --si keeps a lowercase k but uses M, G, T for the rest; -h uses K. 132k versus 128K is what distinguishes them.
• The --total row prints - for both type and mount point, and the percentage is rounded to nearest, not truncated.

Not implemented, and refused rather than silently wrong:

• -i on Windows. No API there exposes inode counts, so it prints df: doesn't support -i option and exits 1, which is what uutils does too. It works on Unix.
• --block-size accepts plain decimal only. 1K, 1M and KB parse as nothing and produce a diagnostic instead of a wrong number.
• --output=FIELD_LIST is not implemented.

Two reference behaviours are deliberately not copied. uutils 0.0.17 on Windows reports Available as 0 and Use% as 100% for every volume, and df C: resolves to the wrong one. Those are defects in the reference, so this port reports the real numbers and the tests do not diff against the rig for those cases; the column layout is pinned against GNU instead, following the same approach unexpand takes.

18 test functions, all green. v fmt -l is clean for everything this changes; it reports src/tr/tr.v, which comes from upstream #239 untouched here.