Fix building with wx 2.8 by rewriting some more string related code.

Replace all calls to .c_str() with .mb_str().

Remove some of the .c_str()/.mb_str() calls where the target is already
wxString.

Move the split()/enum_idx() functions from opts.cpp into
str_split()/vec_find() in strutils.h/strutils.cpp for use in other
files.

Replace the C-style string parsing code in a couple of places in
wxvbam.cpp for processing possible command line options by splitting on
'='.

Also replace a couple of places that use pointer arithmetic in
widgets/joyedit.cpp and widgets/keyedit.cpp with wxString methods.