posix_fallocate() *returns* error codes but does not set errno

Also updates the fhandler_*::ftruncate implementations to adhere to the same
semantics.  The error handling semantics of those syscalls that use
fhandler_*::ftruncate are moved to the implementations of those syscalls (
in particular ftruncate() and friends still set errno and return -1 on error
but that logic is handled in the syscall implementation).

winsup/cygwin/fhandler.cc

@@ -1771,8 +1771,7 @@ fhandler_base::fadvise (off_t offset, off_t length, int advice)
 int
 fhandler_base::ftruncate (off_t length, bool allow_truncate)
 {
-  set_errno (EINVAL);
-  return -1;
+  return EINVAL;
 }
 
 int