The syslog feature allows to run a program and redirect its output/error
stream to system log. It's triggered passing "syslog_stdout" argument to
"Forkhelpers" functions like "execute_command_get_output".
To test this feature use a small preload library to redirect syslog writing.
This allows to test program without changing it.
The log is redirected to /tmp/xyz instead of /dev/log. The name was chosen
to allow for future static build redirection.
The C program is used only for the test, so the code style take into account
this (specifically it does not try to handle all redirect situation and all
error paths).

Signed-off-by: Frediano Ziglio <frediano.ziglio@cloud.com>

Forkexecd was written to avoid some issues with Ocaml and
multi-threading.
Instead use C code to launch processes and avoid these issues.
Interface remains unchanged from Ocaml side but implementation rely
entirely on C code.
vfork() is used to avoid performance memory issue.
Reap of the processes are done directly.
Code automatically reap child processes to avoid zombies.
One small helper is used to better separate Ocaml and C code and
handling syslog redirection. This allows to better debug in
case of issues.
Syslog handling is done in a separate process allowing to restart
the toolstack and keep launched programs running;
note that even with forkexecd daemon one process was used for this
purpose.
Code tries to keep compatibility with forkexecd, in particular:
- SIGPIPE is ignored in the parent;
- /dev/null is open with O_WRONLY even for stdin;
- file descriptors are limited to 1024.
We use close_range (if available) to reduce system calls to close
file descriptors.
Cgroup is set to avoid systemd closing processes on toolstack restart.
There's a fuzzer program to check file remapping algorithm; for this
reason the algorithm is in a separate file.

To turn internal debug on you need to set FORKEXECD_DEBUG_LOGS C
preprocessor macro to 1.

Signed-off-by: Frediano Ziglio <frediano.ziglio@cloud.com>