[0/10] implement support for dwz -m

[Tom Tromey]

Since the 7.5 branch has been made (thanks Joel), I am checking in this
series.

It is largely unchanged since earlier revisions.  I've rebased it a few
times since the last time I posted it; this required a few changes due
to ongoing Fission and .debug_types speedup work.  Jan also reported a
bug after this code was deployed in Rawhide; and that is also fixed in
this series.

The series as a whole was built and regtested on x86-64 Fedora 16.

Tom