Define "onexit" command before "umask"
Add internal "timeout" command

If a command doesn't produce any output forlonger than the timeout
(specified as a floating point number), it is first sent a SIGTERM
signal; if it doesn't react, it is then sent a SIGKILL signal.
Check exit status the same way for internal and external commands
Return void from run_command()

Report all errors in run_command() itself.
Construct run_command() flags differently in main()
Implement --syslog option

This option logs all commands to the system log before they are run;
for commands that fail, the kind of failure (exit with a status != 0,
killed by a signal, timeout) is logged as well.

The environment variable EXXE_IDENT can be used to change how exxe
identifies itself in the syslog.