Set argv[0] to dbus-launch to avoid misleading info from /proc

Set argv[0] to dbus-launch to avoid misleading info from /proc

At previous, argv[0] is the full-qualified path of program, however, if
start dbus-launch failed, it will fall back to find one from $PATH,
while keep the argv[0] as the full-qualified path. So we'll get
misleading info from /proc or ps(1) about dbus-launch process.

One simple solution is just hard-code argv[0] to dbus-launch.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=69716
Reviewed-by: Simon McVittie