maint: simplify parsing df's output in shell scripts

Avoid complicated and error-prone parsing of df's output via
sed(1), cut(1), etc., and instead use df's more modern --output
option.

* src/ioblksize.h (in a comment): Simplify the extraction of the
device name of the mounted file system from df's output.
* tests/dd/skip-seek-past-dev.sh: Likewise.
* tests/du/2g.sh: Likewise for the 'avail' column here.
Also avoid the deprecated use of "tail -NUM".
* tests/misc/stat-mount.sh: While at it, remove the determination
of the mount point of "." via df(1) plus sed(1) as it is unused
since commit v8.5-159-gf57cb37 anyway.  Instead, improve this test
by verifying that the output of "stat -c%m ." at least starts with
a slash '/'.

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