docbook-xsl/scrollkeeper fix

[r0dzilla]

Myself and some other developers had a problem with docbook-xsl failing 
to compile because scrollkeeper, although installed, was missing files 
in /usr/share/xml/scrollkeeper/dtds.

The rebuild-docbook-catalog script would bail out if the files were 
missing, thus bailing out of an otherwise successfull install of the 
module that called it.

After some discussion with other developers, I went about implementing a 
fix that would re-lin scrollkeeper if it was installed, but missing some 
files.  If the file check fails the second time around, it gives up...

This fixed the missing files problem on my computer.

I've modified the docbook-xsl and scrollkeeper modules to use this 
feature (scrollkeeper module actually disables the feature since 
scrollkeeper itself is being compiled).

If you have a module that calls on the rebuild-docbook-catalog in it's 
POST_INSTALL, you can take advantage of this feature by adding a line 
above and below your call to rebuild-docbook-catalog as such:

FORCELINSK="yes"
rebuild-docbook-catalog
unset FORCELINSK

Please feel free to send me any comments/suggestions/criticizims...