LU-9480 lnet: add "lnetctl discover"

LU-9480 lnet: add "lnetctl discover"

Add a "discover" subcommand to lnetctl

jt_discover() in lnetctl.c calls lustre_lnet_discover_nid()
to implement "lnetctl discover". The output is similar to
"lnetctl ping" command.
This patch also does some clean up in linlnetconfig.c
For parameters under global settings, the common code
for them is pulled in funtions ioctl_set_value() and
ioctl_show_global_values().

Test-Parameters: trivial
Signed-off-by: Sonia Sharma <sonia.sharma@intel.com>
Signed-off-by: Amir Shehata <amir.shehata@intel.com>
Signed-off-by: Olaf Weber <olaf@sgi.com>
Change-Id: I98ebb0b27de4b32ea07421f7dd71a4a1c96f3e05
Reviewed-on: https://review.whamcloud.com/25793