layouts: add layouts_p_update_done callback to react on updates in plugins

layouts_p_update_done is a new callback that will be triggered with an array of
entities pointers when an update is performed on a particular layout. This
callback will help layout plugins to perform various consistency checks and/or
update any internal states that should be updated based on the modified entities
information.

No information concerning the udated key/value pairs are provided for now.
For finer-grained controls over what has changed, the plugins will have to use
both the parse_entity callback and the update_done callback to handle what has
changed and what should be rebuilt after the update for now.
layouts/unit: implement a verbose layouts_update_done_callback for testing
parser: enable true/false string parameter with S_P_BOOLEAN

Layouts logic was using true/false instead of yes/no|0/1|up/down
that were supported with S_P_BOOLEAN. Add true/false instead of reverting
to one of the already available parameters as it was strange to not have them.
layouts: add read-only keyspec support

Layout entities Key/value pairs were automatically merged during updates with
the previous logic. Not it is still the case by default but keyspec can now
also be declared as RDONLY in layouts plugins in order to avoid that.

Note that RDONLY keys can still be written by the plugins implementations when
necessary. It only affects the automerge logic avoiding to let administrators
update them with 'scontrol layouts update ...' commands directly.
layouts/unit: add a new readonly key for testing of the new RDONLY keyspec flag