LU-13107 utils: clean up lctl command usage

LU-13107 utils: clean up lctl command usage

The lctl usage is confusing because it lists a number of valid
commands after "testing (DANGEROUS)", such as LFSCK and llog.

Move the useful commands before the "testing" section so it is
not mis-interpreted as all following commands are dangerous.
Group some other commands together with more related commands,
rather than whatever order they happened to be imlpemented in.

Remove function prototypes for commands that no longer exist.

Lustre-change: https://review.whamcloud.com/37108
Lustre-commit: b0efebdaef52d8ac9b02857166ceb00079612ebc

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I469f9c92953762cc46a68e44238c4b67ebacab07
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Stephane Thiell <sthiell@stanford.edu>
Reviewed-on: https://review.whamcloud.com/46637
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>