ugtrain: prevent double game call

In theory it is possible that the game is run twice for dynamic
memory when the game starts but the game pid isn't found. So check
the game call pid there just to be sure.
ugtrain: fix cmd_str_to_cmd_vec() function

The commit 315dcaa1 "ugtrain: never use a shell to run the game"
introduced a bug for commands with more than one space in between.

We need to pass the length to string::substr() and not the end
position. So fix that.

Tested with:
$ ugtrain -P --glc="-s" examples/chromium-bsu32
resulting in:
$ glc-capture -s /usr/games/chromium-bsu &