Fix of multiple \setkern bug

[Lars Hellström <Lars.Hellstrom@math.umu.se>]

I have now completed merging my fix of fontinst's multiple \setkern bug
with the v1.801 fontinst.dtx and have sent the result to Ulrik Vieth. In
addition to the fix of the \setkern bug, there is also three new commands
\resetkern, \unsetkerns, and \ifiskern that do what one would expect them
to.

I hope that a new version of fontinst (with the above fix and additions)
will be out soon.

Lars Hellström