build-sys: add --with-debug-shell=PATH

Distributions may have selinux but not sushell or might
need to set a custom debug shell.

Defaults to /sbin/sushell if selinux is enabled, /bin/sh if not.

[zj: Renamed --with-debugshelltty to --with-debug-tty, and
     added a line in output showing DEBUGSHELL and DEBUGTTY.
     I figure that debug shell is pretty useful, and I hope
     the extra line in configure status will draw attention
     to it.]
coredump: use realloc() loop instead of malloc(768M)

I typically run VMs with 1024MiB allocated; systemd is unable to write
coredumps in this scenario at all because the default kernel
configuration will only overcommit 50% of available RAM.

Avoid this failure by using a realloc() loop.

See: http://lists.freedesktop.org/archives/systemd-devel/2013-April/010709.html