htags could use support for multiple --call-tree --callee-tree options

[Britton Kerin]

so for example:

  htags --suggest --call-tree=foo_prog,foo_call_tree
--call-tree=bar_prog,bar_call_tree

This way the call tree section of the generated web page could be
consistent with the support for multiple main() function and general
way htags --suggest works in which all the source files are analyzed
together.  At present there is a slight inconsistancy within the page
because the source files section shows all the mains but for the
call/callee tree section a single program from the source tree must be
chosen (because cflow ends up with only a single main for a given
invocation).

The alternative is to not used --suggest but instead supply the file
list manually and produce separate pages for different programs that
come from a single code base.  But this is less convenient and
involves a lot of almost-identical pages for source bases that produce
programs that share a lot of common code.

Britton