implement sigaltstack()

[korli]

​http://www.opengroup.org/onlinepubs/000095399/functions/sigaltstack.html

The most common usage of an alternate signal stack is to handle the SIGSEGV signal that is generated if the space available for the normal process stack is exhausted: in this case, a signal handler for SIGSEGV cannot be invoked on the process stack; if we wish to handle it, we must use an alternate signal stack.

Note that BeOS R5 provides set_signal_stack(). Something should be done about it too.

[axeld]

Fixed in hrev21989.

[axeld]

Note that currently, set_signal_stack() is not really BeOS compatible: it will actually only use that new stack for all signals that have the SA_ONSTACK flag set - and it seems this just didn't exist on BeOS.