make "file" use the BFD cache better

make "file" use the BFD cache better

Right now the "file" command will discard the exec_bfd and then
possibly open a new one.

If this ends up reopening the same file, it can cause needless work by
gdb -- destroying all the per-BFD data just to re-read it again.

This patch changes the code to hold a reference to the old exec_bfd
while opening the new one.

The possible downside of this is a higher peak memory use.

2014-02-26  Tom Tromey  <tromey@redhat.com>

	* exec.c (exec_file_attach): Hold a reference to exec_bfd.