Fix a fun bug where, every now again, ego would get its temporary files mangled

Fix a fun bug where, every now again, ego would get its temporary files mangled
and generate invalid calls to the optimisers.

Previously ego would generate a temporary file template that looked like
/tmp/ego.A.BB.XXXXXX, call mktemp() on it to randomise the XXXXXX, and then
replace A and BB with data.

However, it used strrchr to find the A and B. Which would fine, except when
mktemp produced an A or a B in the randomised part...

This code was written on 4 March 1991. I was 16.