make callbacks (subrefs) in maps useful

How callbacks were called before was completly useless -
no context and ldap attribute expected not computed value.
It's hardly backwards incompatible change as there was no
good reason to use them.

Drop flattening nested arrays in maps. Only one level deep
arrays are supported now.

New callbacks are called with enough context to change almost
everything and should return computed value.
make it possible to use sub in Member_Attr

allows to convert members or do other things
document all the changes around callbacks in mappings
sort keys when we process a mapping

So callbacks get reliable set in 'result' argument.
Merge branch 'useful-callbacks'