* kernel/bootstrap.jam: Allow option handler modules to indicate that
regular build process should be skipped.
* options/help.jam: Return 'true', don't EXIT. Existing prevents header
scanning, which is the mechanism that makes 'doc' module work.
* new/doc.jam: Import header scanning rule into global module.
(split-symbol): Handle the case where the are two modules with the same
name (e.g. V1 stlport.jam and V2 stlport.jam).
[SVN r18663]
