Small output modification.

This patch fixes a small output bug where a newline wasn't printed after
an error message.
Remove commented out code.
Suppress zfs header if parameters are invalid.

The script originally printed the zfs header regardless of whether
command line parameters were valid. The script should now abort and
print a message without printing the ZFS header if the -p parameter is
invalid.

This obviates the need for the _call_all function, so it was removed.