[multipathd] add "show wildcards" cli command

[multipathd] add "show wildcards" cli command

The fact I had to look at the code to find the wildcards to use
in "show paths format ...", "show multipath format ..." and
"show pathgroup format ..." was a clear sign that more help was
necessary.

The "show wildcards" command outputs :

multipath format wildcards:
%n  name
%w  uuid
%d  sysfs
%F  failback
%Q  queueing
%N  paths
%r  write_prot
%t  dm-st
%S  size
%f  features
%h  hwhandler
%A  action
%0  path_faults
%1  switch_grp
%2  map_loads
%3  total_q_time
%4  q_timeouts
%s  vend/prod/rev

path format wildcards:
%w  uuid
%i  hcil
%d  dev
%D  dev_t
%t  dm_st
%T  chk_st
%s  vend/prod/rev
%C  next_check
%p  pri
%S  size

pathgroup format wildcards:
%s  selector
%p  pri
%t  dm_st

And for example, "show paths format foo:%d:%S:%i", outs

foo:dev:size:hcil
foo:sda:149G:2:0:0:0