1286601 - Add portable version of ftruncate / SetEndOfFile

Status: NEW

(NSPR :: NSPR, defect)

Description

[Luke Wagner [:luke]]

We currently have at least three places where, for lack of a PR_TruncateOpenFile(), we use PR_FileDesc2NativeHandle() and #ifdef to ftruncate on unix and SetEndOfFile on Windows:
  https://dxr.mozilla.org/mozilla-central/source/netwerk/base/nsFileStreams.cpp#105
  https://dxr.mozilla.org/mozilla-central/source/netwerk/cache/nsDiskCacheDevice.cpp#332
  https://dxr.mozilla.org/mozilla-central/source/netwerk/cache2/CacheFileIOManager.cpp#1922
and I'll probably need to add a fourth, shortly.  It'd be great to add this to NSPR so we could stop duplicating this.