_exit_code behaves differently in terminal vs ZMQ frontends

[takluyver]

The ZMQ frontends (which use pexpect to run !foo shell commands) set _exit_code to the simple exit code of the process.

In the terminal, we use os.system, which on Unix-y systems returns:

exit status indication: a 16-bit number, whose low byte is the signal number that killed the process, and whose high byte is the exit status (if the signal number is zero); the high bit of the low byte is set if a core file was produced.

N.B. the docs also say:

Note that POSIX does not specify the meaning of the return value of the C system() function, so the return value of the Python function is system-dependent.

We should work out which systems this affects (i.e. do BSD & Macs do the same thing?) and do something sensible with the exit code.

[minrk]

OS X:

In [3]: os.system("true")
Out[3]: 0

In [4]: Popen("true", shell=True).wait()
Out[4]: 0

In [5]: os.system("false")
Out[5]: 256

In [6]: Popen("false", shell=True).wait()
Out[6]: 1

In [7]: os.system("notfound")
sh: notfound: command not found
Out[7]: 32512

In [8]: Popen("notfound", shell=True).wait()
/bin/sh: notfound: command not found
Out[8]: 127

Which suggests what you have described elsewhere - the return of os.system is 256 * returncode