Bug using "-" last in `skip-chars-forward'

[Martin Stjernholm]

The following bug exists both in Emacs 21.3 and XEmacs 21.4 (patch
14):

1.  Start (X)Emacs with -q -no-site-file.
2.  Create a buffer and type "-" in it.
3.  Go back to the beginning of the buffer.
4.  Evaluate (skip-chars-forward "y-").

The point does not move, whereas I expected it to skip forward past
the "-". That since a "-" last in a normal regexp character
alternative is not special. `skip-chars-forward' does not use quite
the same syntax, but according to the manual that detail should still
be the same.

Furthermore, if "-" in the buffer is replaced with "y", it still
doesn't skip forward past it.

I.e. when the string to `skip-chars-forward' or `skip-chars-backward'
ends with a "-" that doesn't also start it, both it and the preceding
character are ignored.