This allows Linux's no_new_privs restriction to be disabled when starting
the X server, which may be desirable if xrdp is running inside a kernel
confinement framework such as AppArmor or SELinux.
When allocating a display number, we should be aware that
IANA only allow TCP displays up to :63. This PR adds that restriction in
to sesman.ini as a default, to prevent us allocating unavailable TCP
ports.
By default TCP ports are not enabled for X servers, but users can easily
change this if they wish to access X displays directly over the network.
This restriction is in addition to the MaxSessions limit already present
in sesman.ini
By setting the new config value 'AllowAlternateShell' to 'no' it is now
possible to prevent the use of an alternate shell, which can be set by
the connecting user.
The default remains unchanged and any shell is allowed if the config
value is not specified. It can also be set explicitly to 'yes' to achieve
the same outcome.
Fixes: #850
