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 '/'.

tests/du/2g.sh

@@ -27,10 +27,7 @@ very_expensive_
 
 # Get number of free kilobytes on current partition, so we can
 # skip this test if there is insufficient free space.
-
-# This technique relies on the fact that the 'Available' kilobyte
-# count is the number just before the one with a trailing '%'.
-free_kb=$(df -kP .|tail -1|sed 's/ [0-9][0-9]*%.*//;s/ *$//;s/.* //')
+free_kb=$(df -k --output=avail . | tail -n1)
 case "$free_kb" in
   [0-9]*) ;;
   *) skip_ "invalid size from df: $free_kb";;