LU-4315 doc: split lctl get_param and set_param man pages

LU-4315 doc: split lctl get_param and set_param man pages

Split the "lctl get_param" and "lctl set_param" commands from
"lctl.8" into their own man pages, and improve the description
and examples for each.

Still need to split "lctl list_param" and "lctl conf_param" in
a separate patch.

Test-Parameters: trivial
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Ia03fe707af9bbcaad7a3c4da903d8dfbd23ebbe5
Reviewed-on: https://review.whamcloud.com/35242
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Joseph Gmitter <jgmitter@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Olaf Faaland-LLNL <faaland1@llnl.gov>
Reviewed-by: Oleg Drokin <green@whamcloud.com>