gnetworkmonitornm: Consider NM_STATE_CONNECTED_SITE to be available

`NM_STATE_CONNECTED_SITE` is documented to mean that a default route is
available, but that the internet connectivity check failed. A default
route being available is compatible with the documentation for
GNetworkMonitor:network-available, which should be true if the system
has a default route for at least one of IPv4 and IPv6.

https://developer.gnome.org/NetworkManager/stable/nm-dbus-types.html

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Fixes: #1788
9 jobs for 1788-network-more-reachable in 24 minutes and 13 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #314209
cross-android_api21_arm64

00:03:07

passed #314210
cross-android_api28_arm64

00:01:28

passed #314211
cross-mingw64

00:02:07

passed #314207
debian-stable-x86_64

00:05:51

passed #314206
fedora-x86_64

00:09:44

passed #314208
G_DISABLE_ASSERT

00:02:09

passed #314212
win32
msys2-mingw32

00:14:04

passed #314213
win32
vs2017-x64

00:09:22

 
  Coverage
passed #314214
coverage

00:01:18

78.0%