package/busybox/udhcpc.script: ensure action argument is correctly passed to hook scripts

commit f79a4208 (package/busybox/udhcpc.script: support RFC3442
static routes) used 'set --' clobbering the positional arguments, causing
the action argument to not be correctly forwarded to hook scripts for the
renew / bound cases if static routes are provided by the server.

As a workaround, save the action argument at the beginning of the script and
use that when calling hook scripts.

Reported-by: 王琦 <wangwangqi2011@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>