Generate "Unsupported socket function" stubs using PL_ppaddr.

Generate "Unsupported socket function" stubs using PL_ppaddr.

Instead of having each socket op conditionally compile as either the
implementation or a DIE() depending on #HAS_SOCKET

1: remove the conditional code from the ops themselves
2: only compile the ops if HAS_SOCKET is defined
3: general conditional code for the intialisation of PL_ppaddr - as appropriate
   either the ops, or Perl_unimplemented_op
4: Amend Perl_unimplemented_op to generate the appropriate DIE() for socket
   ops (ie not the "panic"... message)

Whilst this complicates the support code in regen/opcode.pl, it's already a
net saving of 5 lines in the C code.