I've added a new API function, sv_set_undef()

I hope the following commit is self-explanatory and non-controversial:

commit e03e82a04dda6a44b815f3367c69afb0e953dc91
Author:     David Mitchell <davem@iabyn.com>
AuthorDate: Thu Nov 24 09:40:44 2016 +0000
Commit:     David Mitchell <davem@iabyn.com>
CommitDate: Thu Nov 24 13:42:22 2016 +0000

    add sv_set_undef() API function
    
    This function is equivalent to sv_setsv(sv, &PL_sv_undef), but more
    efficient.
    
    Also change the obvious places in the core to use the new idiom.


-- 
"There's something wrong with our bloody ships today, Chatfield."
    -- Admiral Beatty at the Battle of Jutland, 31st May 1916.

• I've added a new API function, sv_set_undef() by Dave Mitchell