scripts: add --user-network to qemustart

scripts: add --user-network to qemustart

qemustart is a handy script to quickly test OpenWrt firmware using
qemu.  Bringing up networking currently requires a bridge-helper
setup with privileged IP and bridge assignment.  To simplify
testing scenarios like the package manager, which need both shell
access and outbound internet, add a user-mode networking option
backed by SLIRP that requires no privileges.

To stay backward compatible, the defaults don't change.  The new
flag --user-network attaches two NICs (LAN + WAN) and forwards
three host ports to the guest LAN interface (192.168.1.1):
2222 -> 22 (ssh), 8080 -> 80 (http) and 8443 -> 443 (https).  The
host-side ports can be overridden with --ssh-port, --http-port
and --https-port.

Link: https://github.com/openwrt/openwrt/pull/23424
Signed-off-by: Paul Spooren <mail@aparcar.org>