[RFD] objfiles.h, symfile.c, mdebugread.c: Fixes for Digital Unix.

[Jim Blandy]

It seems like we're adding a bunch of members to struct objfile, and a
bunch of macros to access them, and a bunch of code to
default_symfile_offsets to initialize them, for the benefit of
a new function `sect_index_for_sc' in mdebugread.c.

It seems like it would be cleaner to simply have some mdebug-specific
code look up the offsets and fill in a table indexed by mdebug SC
number, which sect_index_for_sc could then access directly.  That
would keep the entire change isolated in mdebugread.c, which is the
only reader that needs all that info.  Does that sound reasonable?

Peter, I think you're more familiar with the big picture regarding the
section offset handling than I am, so I'm interested in your comments.