doc: Remove reference to general signal handling

Signed-off-by: Jens Georg <mail@jensge.org>

https://bugzilla.gnome.org/show_bug.cgi?id=729827
Bump glib requirement to 2.28
Fix warning with clang
Prevent critical if there's no system bus

Found by Juan A. Suarez Romero <jasuarez@igalia.com>

Signed-off-by: Jens Georg <mail@jensge.org>

https://bugzilla.gnome.org/show_bug.cgi?id=727709
Check for system bus before using DBus

If there's no system bus, there's no need to check for Connman or
NetworkManager availability.

Signed-off-by: Jens Georg <mail@jensge.org>

https://bugzilla.gnome.org/show_bug.cgi?id=727709