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

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.]