lfp_sendfile: don't return EAGAIN in case of partial write

In order to match the semantics of the Linux sendfile(), the wrapper
now hides EAGAIN if a partial write (more than 0 bytes, but fewer then
requested) was performed.

src/lib/sendfile.c

@@ -48,14 +48,18 @@ lfp_sendfile(int out_fd, int in_fd, off_t offset, size_t nbytes)
     off_t off = offset;
     return (ssize_t) sendfile(out_fd, in_fd, &off, nbytes);
 # elif defined(__FreeBSD__) || defined(__DragonFly__)
-    off_t sbytes;
+    off_t sbytes = 0;
     int res = sendfile(in_fd, out_fd, offset, nbytes, NULL, &sbytes, 0);
-    if (res == 0) { return sbytes; }
-    return res;
+    if (res == 0 || (res < 0 && errno == EAGAIN && sbytes > 0)) {
+        return sbytes;
+    }
+    return -1;
 # elif defined(__APPLE__)
     off_t len = nbytes;
     int res = sendfile(in_fd, out_fd, offset, &len, NULL, 0);
-    if (res == 0) { return len; }
+    if (res == 0 || (res < 0 && errno == EAGAIN && len > 0)) {
+        return len;
+    }
     return -1;
 # else
 #  error "It appears that this OS has sendfile(), but LFP doesn't use it at the moment"