new options for hurd crash server

[Roland McGrath <roland@frob.com>]

I've added some options to the Hurd crash server (/hurd/crash),
that takes over when a program crashes and wants to "dump core".

It now has three possible modes of operation, selected by command line
switches to the translator.

		-s/--suspend	suspend crashing process (old behavior)
		-k/--kill	kill crashing process, no core dump
		-c/--core-file	dump core file

The --core-file option does not in fact dump a core file, that is still not
supported (yet).  

You can choose the mode of operation by how you set the passive translator:

	# settrans -cg /servers/crash /hurd/crash --kill

You can also set (and query) the mode of the running crash server by using
fsysopts: 

	# fsysopts /servers/crash
	/hurd/crash --suspend
	# fsysopts /servers/crash --kill
	# fsysopts /servers/crash 
	/hurd/crash --kill

Theoretically one can select the crash server naming point programs will
use with the CRASHSERVER environment variable.  But that does not appear to
be working.  If it were, then I would suggest having the default
installation make three naming points for the three modes of operation:

	# settrans -c /servers/crash-suspend	/hurd/crash --suspend
	# settrans -c /servers/crash-kill	/hurd/crash --kill
	# settrans -c /servers/crash-dump-core	/hurd/crash --dump-core

and then /servers/crash can be a symlink to one of these, to choose the
system-wide default behavior.  Users can use CRASHSERVER=/servers/crash-HOW
in their environment to select the desired option for particular uses.

But, that does not appear to be working, so right now until we get around
to fixing that bug, it's /servers/crash or nothing and the system-wide
default is the only choice.