Change the policy for parsing underspecified params strings.

Change the policy for parsing underspecified params strings.

In conversation with a user last week, it emerged that the command
'solo --generate 1 9jk#12345' was giving a different game from the one
it gave when I ran it, and it turns out that this is because I've set
SOLO_DEFAULT=7jxdi in my environment to make GUI Solo automatically
start up in my (current) favourite mode. And the difficulty setting
from that parameter string was being reused to fill in the unspecified
difficulty slot in the '9jk', so that the same params string was being
interpreted differently by our two machines.

This is certainly wrong - the whole point of random seed strings like
that is to be interpreted the same way everywhere. But it's a side
effect of something I did do on purpose, for people switching back and
forth between playing randomly generated games and playing a game id
pasted (or typed) in from elsewhere. So this fix, with a giant comment
explaining it, I _think_ should retain the behaviour I originally
wanted while getting rid of the behaviour I didn't.