Described in the comments and documented the semantics of the parameters and

return values of wxMBConv methods and tried to make them more consistent. The
only (intentional) backwards incompatible change is that cMB2WC/cWC2MB now
return the length of the converted string in outLen parameter and not length+1

Added wxMBConv::GetMBNul() and use it instead of supposing that all multibyte
strings are always terminated with a single NUL which is wrong for UTF-16/32.

Using GetMBNul(), completely rewrote cMB2WC/cWC2MB() to accept a string of the
specified length, whether it is NUL-terminated or not. This means that they
don't overwrite the provided buffer any more and convert the entire string in
all cases.

Fixed bug in wxMBConvUTF16::WC2MB() which didn't NUL-terminate the string
properlyv even if there was enough space.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@38498 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775