The previously-posted implementation of named references assumes all
PM_NAMEREF parameters are created as simple scalars with the referent
parameter name stored in the u.str field of the referencing param
structure.  This is efficient but complicates the creation of special
named references, for example in modules.

This patch changes the implementation to call the get/set/unset
function pointers when the PM_SPECIAL flag is present, and otherwise
to continue directly accessing u.str.  This should be applied after
the namespace-syntax patch, though if wanted independently there is
only a single line of overlap with an obvious adjustment.

Included is a fix to builtins.yo to add an explicit summary line for
"typeset -n".