As far as possible, use of the X11 display number is kept to
X11-specific routines. This is to make it easier to restructure
the code to add non-X11 display support.
This commit addresses these kind of errors:
portability: Passing NULL after the last typed argument to a variadic function leads to undefined behaviour. [varFuncNullUB]
Reason is that C does not guarantee that all pointer types are the same
size. See C99 6.2.5(27). cppcheck requires some sort of cast when NULL
is used as the last argument in a variadic list.
env.c : The value of XRDP_SESSION in the environment is now set to the
PID of the sesexec process, which ties up the session with the
output of "xrdp-sesadmin -c=list".
Later versions of xrdp-sesadmin can use this value to get
information about the current process.