LU-16631 llmount: improve usability

LU-16631 llmount: improve usability

Add some simple help messages to both llmount.sh and
llmountcleanup.sh, similar to what auster has. This
will help unfamilar people understand the use of these
scripts.

Add option to disable client setup for llmount.sh. Add
options for llmount.sh environment variables.

Fix a couple small shellcheck warnings.

Update the file headers to have the SPDX license and
use the standard format.

Test-Parameters: trivial
Signed-off-by: Timothy Day <timday@amazon.com>
Change-Id: I50dbb30bad8c8bc0479585293056d61a25aa001d
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/50260
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>