Remove basename(1). Clean up/shorten some coreutils pipelines

Basenames that remain, in cmd/zed/zed.d/statechange-led.sh:
dev=$(basename "$(echo "$therest" | awk '{print $(NF-1)}')")
vdev=$(basename "$ZEVENT_VDEV_PATH")
I don't wanna interfere with #11988

scripts/zfs-tests.sh:
SINGLETESTFILE=$(basename "$SINGLETEST")
tests/zfs-tests/tests/functional/cli_user/zfs_list/zfs_list.kshlib:
ACTUAL=$(basename $dataset)
ACTUAL=$(basename $dataset)
tests/zfs-tests/tests/functional/cli_user/zpool_iostat/
zpool_iostat_-c_homedir.ksh:
typeset USER_SCRIPT=$(basename "$USER_SCRIPT_FULL")
tests/zfs-tests/tests/functional/cli_user/zpool_iostat/
zpool_iostat_-c_searchpath.ksh:
typeset CMD_1=$(basename "$SCRIPT_1")
typeset CMD_2=$(basename "$SCRIPT_2")
tests/zfs-tests/tests/functional/cli_user/zpool_status/
zpool_status_-c_homedir.ksh:
typeset USER_SCRIPT=$(basename "$USER_SCRIPT_FULL")
tests/zfs-tests/tests/functional/cli_user/zpool_status/
zpool_status_-c_searchpath.ksh
typeset CMD_1=$(basename "$SCRIPT_1")
typeset CMD_2=$(basename "$SCRIPT_2")
tests/zfs-tests/tests/functional/migration/migration.cfg:
export BNAME=basename $TESTFILE
tests/zfs-tests/tests/perf/perf.shlib:
typeset logbase="$(get_perf_output_dir)/$(basename \
tests/zfs-tests/tests/perf/perf.shlib:
typeset logbase="$(get_perf_output_dir)/$(basename \

These are potentially Of Directories, where basename is actually
useful

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #12652