sftp: route Recv* parsers through GetStringRef

Convert the unflagged server Recv* handlers (RecvRealPath, RecvRMDIR,
RecvMKDIR, RecvOpenDir, RecvReadDir, RecvClose, RecvRemove, RecvFSTAT,
RecvSTAT, RecvLSTAT, RecvSetSTAT, RecvFSetSTAT) to GetStringRef,
GetSize, and GetUint32 so all Recv* paths share one bounds-checked
idiom.

Author: ejohnstown