Fix storage of values of engine string options

Fix storage of values of engine string options

Originally all option definitions given in option features were stored
in a static array of generous size (MSG_SIZ). But this was changed to
allocated memory for not wasting space on unused option slots. The allocated
memory wasonly large enough to hold the default value of the option,
which could very well be an empty string. Changing the option setting
to a longer strig would then produce memory corruption through buffer overrun.
Now the memory allocated for option features is again at least MSG_SIZ.