gdb/coffread: bring separate debug file logic into line with elfread.c

In this commit:

  commit 8a92335b
  Date:   Fri Feb 1 19:39:04 2013 +0000

the logic for when we try to load a separate debug file in elfread.c
was extended.  The new code checks that the objfile doesn't already
have a separate debug objfile linked to it, and that the objfile isn't
itself a separate debug objfile for some other objfile.

The coffread code wasn't extended at the same time.

I don't know if it's possible for the coffread code to get into the
same state where these checks are needed, but I don't see why having
these checks would be a problem.  In a later commit I plan to merge
this part of the elfread and coffread code, so bringing these two
pieces of code into line first makes that job easier.

I've tested this with a simple test binary compiled with the mingw
toolchain on a Linux host.  After compiling the binary and splitting
out the debug info GDB still finds and loads the separate debug info.

Approved-By: Tom Tromey <tom@tromey.com>