[multipathd] add "show maps format ..." cli command

[multipathd] add "show maps format ..." cli command

Just like "show paths format ...", it gives users more control
over the report format of multipaths information.

Example:
$ sudo multipathd -k'show maps format "%n %s %S %d %t %r %Q %N"'
name              vend/prod/rev    size sysfs dm-st  write_prot queueing paths
353333330000007d0 Linux,scsi_debug 8.0M dm-1  active rw         -        2
353333330000007d1 Linux,scsi_debug 8.0M dm-2  active rw         -        2
35333333000000bb8 Linux,scsi_debug 8.0M dm-3  active rw         -        2
35333333000000bb9 Linux,scsi_debug 8.0M dm-4  active rw         -        2