The size of the listen_port for sesman cannot exceed
XRDP_SOCKETS_MAXPATH. We should use this value rather than
an arbitrary value of 256.

The sesman tools have some private functions to make syncronous calls
to sesman over SCP. This commit moves these calls to a new module
scp_sync in libipm so that they can be utilised by other parts of
xrdp (i.e. chansrv)

THe SCP interface is modified so that a program can connect to
sesman and ask for the sockdir to be created for the currently
authenticated user

Chansrv now checks for the user sockdir being present. If it
isn't, it connects to chansrv and requests it be created.

This also needs the sesman port to be added to the chansrv
config struct.

The code to determine the socket address of chansrv when using
a manually started xrdp-chansrv may need some help determining
the UID of the session.

This commit allows a UID to be optionally specified in the
DISPLAY() function, if the code is unable to determine the
UID automatically from the connection parameters.

If a manual chansrvport is entered, xrdp now logs what it is
connecting to, to assist in debugging.