Fixed some warnings.
Introduced constant for end of ME command prefix index.
Implemented (approximate) computation of overhead to get most nicks in single ISON query.

The computation is approximate because I've added 10 chars overhead in
the computation, since without the 10 chars there were some spurious
nicks missing even though these nicks were online. The number 10 was
arbitrarily chosen. It seems to do the trick, though.