minor, make pmml_socket() 1-adic, lose the vararg functionality

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
fix, gcc, gcc cannot cope with pointers to unions of flexible arrays, so use the array of size 0 trick

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
Merge branch 'code/hygiene' into next

* code/hygiene:
  fix, gcc, gcc cannot cope with pointers to unions of flexible arrays, so use the array of size 0 trick
  minor, make pmml_socket() 1-adic, lose the vararg functionality