[multipathd] add "show config" CLI command

[multipathd] add "show config" CLI command

This commands dumps a multipath.conf config file.

The "defaults" section is factorized : settings in sync with the internal
defaults are not printed.

The "devices" section lists the internal hwtable, plus mods and adds from
the actual config file. The layout is factorized : settings in sync with
the "defaults" section are not printed.

The "devnode_blacklist" section is ommited for now, as this requires a bit
more infrastructure.

The "multipaths" section is not factorized as we can not reliably guess the
hardware a multipath is parented to. Multipath entries are just clones
of their actual config file equivalent.