Convert array_multisort to HNI

Summary: fixes #3993

Also removed all MixedVariableArugument flags because it's no longer used

Reviewed By: @sgolemon, @JoelMarcey

Differential Revision: D1621857

Signature: t1:1621857:1414521500:d1f977cecba55458b5e7c92f6c79a303a52f0f4c
Convert POSIX to HNI

Summary: fixes #3933

Reviewed By: @markw65

Differential Revision: D1604628

Signature: t1:1604628:1414799212:5aefd092bdc9a672b65f681b9597a7ef7b6c214b
Convert DOMDocument to HNI

Summary: fixes #3745

Reviewed By: @ptarjan, @JoelMarcey

Differential Revision: D1638235

Signature: t1:1638235:1414612822:fd550de38c1167d73edc2c0e7cfbb9cb6cbc058c