Restructure session start

[matt335672]

This is now ready for review, following my own testing.

This is a major change to sesman.

There are quite a few file changes, bu the main implementation points are:-

1. sesman/session.c is split into two files
   a) session_list.c contains all the list-handling code.
   b) session.c now contains just the code to manage a single session.
2. session starting logic is reworked to make it possible to detect X server failures as a separate class of error

I've split session.c into two with #1961 in mind - I'm getting close to the point where I can split sesman entirely. session_list.c contains all the code for sesman and session.c contains all the code for sesexec.

The reworking of session starting logic allows me to use @derekschrock's waitforx utility to check for a working X server before starting the session. If the X server isn't started, this is reported back to xrdp.

Not only is the log file more readable with this change, but error reporting is improved. If the X server can't be started, the logging window now looks like this, rather than blue-screening for a minute or so:-

Also sesman logging is vastly improved on starting a session. Here's a development logging example:-

2023-03-13T21:36:13.871+0000] [INFO ] [g_sck_accept(os_calls.c:1297)] Socket 13: connection accepted from AF_UNIX
[2023-03-13T21:36:13.980+0000] [INFO ] [process_sys_login_request(scp_process.c:296)] Received system login request from xrdp for user: testuser IP: ::ffff:1.1.1.1
[2023-03-13T21:36:14.400+0000] [INFO ] [verify_pam_conv(verify_user_pam.c:138)] Handling struct pam_message { style = PAM_PROMPT_ECHO_OFF, msg = "Password: " }
[2023-03-13T21:36:14.198+0000] [INFO ] [access_login_allowed(access.c:56)] Terminal Server Users group is disabled, allowing authentication
[2023-03-13T21:36:14.246+0000] [INFO ] [authenticate_and_authorize_connection(scp_process.c:160)] Access permitted for user=testuser uid=1001
[2023-03-13T21:36:14.330+0000] [INFO ] [process_create_session_request(scp_process.c:483)] Received request from xrdp to create a session for user testuser type=Xorg geometry=1920x1080, bpp=24, shell="", dir=""
[2023-03-13T21:36:14.374+0000] [INFO ] [session_start(session.c:836)] calling auth_start_session for uid=1001 from pid 1621183
[2023-03-13T21:36:14.722+0000] [INFO ] [start_x_server(session.c:500)] Starting X server on display 10: /usr/lib/xorg/Xorg :10 -auth .Xauthority -config xrdp/xorg.conf -noreset -nolisten tcp -logfile .xorgxrdp.%s.log
[2023-03-13T21:36:15.819+0000] [INFO ] [session_start_subprocess(session.c:774)] X server :10 is working
[2023-03-13T21:36:15.867+0000] [INFO ] [session_start_subprocess(session.c:775)] Starting window manager for display :10
[2023-03-13T21:36:15.910+0000] [INFO ] [session_start_subprocess(session.c:786)] Starting the xrdp channel server for display :10
[2023-03-13T21:36:15.912+0000] [INFO ] [start_window_manager(session.c:292)] Using the default window manager on display 10: /etc/xrdp/startwm.sh
[2023-03-13T21:36:15.969+0000] [INFO ] [allocate_and_start_session(scp_process.c:247)] ++ created session: username testuser, ip ::ffff:172.19.73.10
[2023-03-13T21:36:15.970+0000] [INFO ] [run_xrdp_session(session.c:556)] Session in progress on display :10. Waiting until the window manager (pid 1621440) exits to end the session

and at session end:-

2023-03-13T21:36:37.351+0000] [INFO ] [run_xrdp_session(session.c:564)] Window manager (pid 1621440, display 10) finished normally in 21 secs
[2023-03-13T21:36:38.250+0000] [INFO ] [run_xrdp_session(session.c:596)] Terminating X server (pid 1621263) on display :10
[2023-03-13T21:36:38.322+0000] [INFO ] [run_xrdp_session(session.c:602)] Terminating the xrdp channel server (pid 1621441) on display :10
[2023-03-13T21:36:38.461+0000] [INFO ] [run_xrdp_session(session.c:609)] X server pid 1621263 on display :10 finished
[2023-03-13T21:36:38.700+0000] [INFO ] [run_xrdp_session(session.c:615)] xrdp channel server pid 1621441 on display :10 finished
[2023-03-13T21:36:38.778+0000] [INFO ] [cleanup_sockets(session.c:136)] cleanup_sockets:
[2023-03-13T21:36:38.841+0000] [INFO ] [session_start(session.c:844)] Calling auth_stop_session from pid 1621183
[2023-03-13T21:36:38.984+0000] [INFO ] [sig_sesman_session_end(sig.c:112)] Process 1621183 has exited
[2023-03-13T21:36:39.494+0000] [INFO ] [session_kill(session_list.c:382)] Calling auth_end for pid 1621183 from pid 1502282
[2023-03-13T21:36:39.133+0000] [INFO ] [session_kill(session_list.c:388)] ++ terminated session: UID 1001 (testuser), display :10.0, session_pid 1621183, ip ::ffff:172.19.73.10