Extend value_match to extract an asserted value from a full value

Extend value_match to extract an asserted value from a full value
if needed.  This is controlled by SLAP_MR_VALUE_IS_IN_MR_SYNTAX,
a new flag that should be set when evaluating filters such as in
searches and compares and unset otherwise (such as in modify).

Now, some callers of value_match, notably value_find, don't know
whether to set it or not.  We'll see to that.