exec requiem.cfg even if no autoexec.cfg

Resolves issue #29.

requiem.cfg is intended to be exec'd after autoexec.cfg. The solution to
issue #29 is to perform this explicit exec even if there is no autoexec.cfg...
do it after the failed attempt.

The code is a bit of if-spaghetti here so I didn't want to refactor it. I
particularly don't want to disturb the management of the hunk allocations.
This means a bit of code-duplication in the solution, but it's minor.

I preserved the existing behavior of only doing this implicit exec once (via
the id1_autoexec_done flag). I.e. after starting Quake, if you manually type
"exec autoexec.cfg" into the console then it will exec autoexec.cfg again (or
try to), but it's not going to re-exec requiem.cfg afterward. Of course you
can manually exec requiem.cfg as many times as you want.