[PATCH 10/17] layouts: add read-only keyspec support

Layout entities Key/value pairs were automatically merged during updates with
the previous logic. 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.