sel4runtime should provide default exit handlers instead of raising a cap fault.

[kent-mcleod]

It is less confusing if the exit handler printed out an error message before cap faulting, or provided an alternative default implementation that suspends the current thread if an initial_thread_tcb is saved in the current runtime environment before triggering a fault. The current implementation:

    /* If the exit is never set this will try and call a NULL function
     * pointer which should result in a fault. This is as good a way as
     * any to exit the process if we don't know anything better about
     * the environment. */
    env.exit_cb(code);

Will raise this fault:

 Caught cap fault in send phase at address 0
while trying to handle:
vm fault on data at address 0 with status 0x4
in thread 0xffffff801fe08400 "rootserver" at address 0
With stack:
0x41ce98: 0x401740
0x41cea0: 0x0
0x41cea8: 0x401100
0x41ceb0: 0x41cef0
0x41ceb8: 0x40121b
0x41cec0: 0x0
0x41cec8: 0x41cf30
0x41ced0: 0x41cf20
0x41ced8: 0x41cf10
0x41cee0: 0x1
0x41cee8: 0x4010f2
0x41cef0: 0x41cff0
0x41cef8: 0x401394
0x41cf00: 0x0
0x41cf08: 0x525000
0x41cf10: 0x41201c