Correct the column header printed by `df` when the `--block-size`
argument has a value that is a multiple of 1024. After this commit,
the header looks like "1K" or "4M" or "117G", etc., depending on the
particular value of the block size. For example:
$ df --block-size=1024 | head -n1
Filesystem 1K-blocks Used Available Use% Mounted on
$ df --block-size=2048 | head -n1
Filesystem 2K-blocks Used Available Use% Mounted on
$ df --block-size=3072 | head -n1
Filesystem 3K-blocks Used Available Use% Mounted on
$ df --block-size=4096 | head -n1
Filesystem 4K-blocks Used Available Use% Mounted on
Add support for the `--total` option to `df`, which displays the total
of each numeric column. For example,
$ df --total
Filesystem 1K-blocks Used Available Use% Mounted on
udev 3858016 0 3858016 0% /dev
...
/dev/loop14 63488 63488 0 100% /snap/core20/1361
total 258775268 98099712 148220200 40% -
Change the `filter_mount_list()` function so that it always produces
the same order of `MountInfo` objects. This change ultimately results
in `df` printing its table of filesystems in the same order on each
execution. Previously, the table was in an arbitrary order because the
`MountInfo` objects were read from a `HashMap`.
Fixes#3086.
