Add ftruncate(2).

The claim that "the port library" modifies the file offset appears to only ever
have been true of Harmony's Win32 implementation. Certainly, this is not the
behavior of ftruncate(2), so we can lose the cruft.

Bug: 3107501
Change-Id: I35092fd986296b78c4847c7215765f9997339f37
13 files changed