kdump-load: Add usage/help messages to this script

The kdump-load script is the only one that has more than a
single functionality, potentially being invoked in different ways.

With that in mind, just add a small usage/help for users'
information (thanks Emil for the suggestion).

While at it, changed the additional commands to be more friendly
and require exactly one command to be passed - changes summary:
s/initrd/create-initrd
s/clear/clear-initrd
s//load

Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>

kdump-load.header

@@ -1,5 +1,5 @@
 #!/bin/bash
-set -uo pipefail
+set -o pipefail  #  Didn't set -u on purpose, since users could pass empty $1
 #
 #  SPDX-License-Identifier: LGPL-2.1+
 #