Clean up (k)shlib and cfg file shebangs

Most kshlib files are imported by other scripts
and do not have a shebang at the top of their files.
Make all kshlib follow this convention.

Remove shebangs from cfg files as well.

Reviewed-by: loli10K <ezomori.nozomu@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Close #7406

tests/zfs-tests/include/libtest.shlib

@@ -1,4 +1,3 @@
-#!/bin/ksh -p
 #
 # CDDL HEADER START
 #

tests/zfs-tests/include/zpool_script.shlib

@@ -1,4 +1,3 @@
-#!/bin/ksh -p
 #
 # Common functions used by the zpool_status and zpool_iostat tests for running
 # scripts with the -c option.