preamble: debug if the inferior is still running

the previous breadcrumbs didn't really reveal anything - as far as gdb's
api is concerned everything is dandy. I have a new theory though: the
process gets restarted out from under gdb (IOW the inferior disappears
after it has been attached). to verify this theory we now log whether
the inferior's pid exists. this is a bit subject to racing but should be
good enough for debugging purposes