[perl #107366] Allow attributes to set :lvalue on defined sub

[perl #107366] Allow attributes to set :lvalue on defined sub

This provides enough rope for those who want to hang themselves, and
also for those who know how to use the rope without hanging them-
selves. :-)

Since this is not generally a reliable thing to be doing, a warning is emitted whenever :lvalue is turned on or off on a defined subroutine.

But attributes.pm will flip the flag anyway.  :lvalue in a sub declar-
ation still refuses to modify a defined Perl sub, as before.