Still many bugs in mhl/*.h

[Roland Illig]

Whenever I browse through the files in the new mhl/ directory, I stumble
across lots of bugs. There are calls to isdigit(char) instead of the
required isdigit(unsigned char), many typos, a buffer overflow in
__mhl_str_concat_hlp, and probably many more.

I did this in the branch origin/mc-4.6. Isn't that the one that is
supposed to form the release? If it is, please fix the code before
releasing.

Roland