new feature: drop to gdb on crash if run interactively

On a crash, if the process is running interactively, it would really be cool to drop to gdb first to give the user a chance to poke around before the process goes away. This forks and execs gdb on a signal if a config option (EIKI_GDB_IF_TTY) is defined. Tested in x86oo. Feel free to take or leave, I'm just having fun here :-)