Third part of fix for bug #8679 - recvfile code path using splice() on Linux leaves...

Third part of fix for bug #8679 - recvfile code path using splice() on Linux leaves data in the pipe on short write.

Fix default_sys_recvfile() to correctly cope with
short writes. Return the amount written. Return
-1 and set errno if no data could be written.
(cherry picked from commit 5e6263960aaf1a5f9993cb7bb5646d36ff92b9cc)